[1 of 3] Support long raw TUI goal objectives (#27508)

## Stack

1. **[1 of 3] Support long raw TUI goal objectives** - this PR
2. [2 of 3] Support long pasted text in TUI goals - #27509
3. [3 of 3] Support images in TUI goals - #27510

## Why

`thread/goal/set` limits persisted objective text to 4000 characters.
The TUI used to reject raw `/goal` objectives above that limit, even
though the client can make them usable by writing the long text to a
file and storing a short objective that points at that file.

This also needs to work for remote app-server sessions: filesystem API
calls must create files on the app-server host, and the stored path must
be meaningful to the agent on that host.

## What Changed

- Adds an app-server-host path helper so TUI code can build paths that
are resolved on the app-server host rather than the TUI host.
- Adds TUI app-server session helpers for `fs/createDirectory`,
`fs/writeFile`, `fs/readFile`, and `fs/remove` that work for embedded
and remote app-server sessions without changing the app-server protocol.
- Materializes oversized raw `/goal` objectives into
`$CODEX_HOME/attachments/<uuid>/goal-objective.md` through the
app-server filesystem APIs, then stores a short, readable objective that
directs the agent to that file.
- Reads managed objective files back for `/goal edit`. Other goal UI
renders the readable stored objective normally, without
managed-file-specific presentation logic.
- Recognizes managed references only when they name the expected
generated file under the app server's reported `$CODEX_HOME`, and cleans
up newly materialized files when goal replacement or setting does not
complete.

## Verification

- Added/updated TUI tests for raw oversized `/goal` submission, large
inline-paste expansion, queued oversized goals, app-facing
materialization before `thread/goal/set`, managed-path validation,
editing, and cleanup.
- Added/updated app-server-client remote coverage for initialized remote
Codex home handling.

## Manual Testing

- Ran the real TUI against a Unix-socket app server with different local
and server `$CODEX_HOME` directories. Oversized goals wrote only under
the server home, and persisted references used the server-canonical path
rather than the TUI path.
- Exercised 3,999-, 4,000-, and 4,001-character raw objectives. The
first two stayed inline without new files; the 4,001-character objective
became a managed objective file.
- Submitted a larger 8,275-character objective, verified its full
contents on the app-server host, and observed the goal continuation open
the referenced server-side file.
- Opened `/goal edit` for a managed objective and verified the full text
was restored through remote `fs/readFile`.
- Submitted an oversized replacement while a goal was active, verified
no file was written before confirmation, then canceled and confirmed
that the existing goal and attachment count were unchanged.

codex-rs/tui/src/chatwidget/slash_dispatch.rs

@@ -5,7 +5,6 @@
 //! dispatch step and records the staged entry once the command has been handled, so
 //! slash-command recall follows the same submitted-input rule as ordinary text.
 
-use super::goal_validation::GoalObjectiveValidationSource;
 use super::*;
 use crate::app_event::ThreadGoalSetMode;
 use crate::bottom_pane::prompt_args::parse_slash_name;
@@ -570,12 +569,6 @@ impl ChatWidget {
             return;
         }
 
-        if cmd == SlashCommand::Goal
-            && !self.goal_objective_with_pending_pastes_is_allowed(&args, &text_elements)
-        {
-            return;
-        }
-
         let Some((prepared_args, prepared_elements)) =
             self.prepare_live_inline_args(args, text_elements)
         else {
@@ -607,6 +600,12 @@ impl ChatWidget {
         }
     }
 
+    fn clear_live_goal_submission(&mut self) {
+        self.bottom_pane
+            .set_composer_text(String::new(), Vec::new(), Vec::new());
+        self.bottom_pane.drain_pending_submission_state();
+    }
+
     fn prepared_inline_user_message(
         &mut self,
         args: String,
@@ -714,6 +713,9 @@ impl ChatWidget {
             }
             SlashCommand::Goal if !trimmed.is_empty() => {
                 if !self.config.features.enabled(Feature::Goals) {
+                    if source == SlashCommandDispatchSource::Live {
+                        self.clear_live_goal_submission();
+                    }
                     return;
                 }
                 enum GoalControlCommand {
@@ -727,7 +729,7 @@ impl ChatWidget {
                             thread_id: self.thread_id,
                         });
                         if source == SlashCommandDispatchSource::Live {
-                            self.bottom_pane.drain_pending_submission_state();
+                            self.clear_live_goal_submission();
                         }
                         return;
                     }
@@ -743,6 +745,9 @@ impl ChatWidget {
                                 "The session must start before you can change a goal.".to_string(),
                             ),
                         );
+                        if source == SlashCommandDispatchSource::Live {
+                            self.clear_live_goal_submission();
+                        }
                         return;
                     };
                     match command {
@@ -757,29 +762,11 @@ impl ChatWidget {
                     }
                     self.append_message_history_entry(format!("/goal {trimmed}"));
                     if source == SlashCommandDispatchSource::Live {
-                        self.bottom_pane.drain_pending_submission_state();
+                        self.clear_live_goal_submission();
                     }
                     return;
                 }
                 let objective = args.trim();
-                if objective.is_empty() {
-                    self.add_error_message("Goal objective must not be empty.".to_string());
-                    self.add_info_message(
-                        GOAL_USAGE.to_string(),
-                        Some(GOAL_USAGE_HINT.to_string()),
-                    );
-                    if source == SlashCommandDispatchSource::Live {
-                        self.bottom_pane.drain_pending_submission_state();
-                    }
-                    return;
-                }
-                let validation_source = match source {
-                    SlashCommandDispatchSource::Live => GoalObjectiveValidationSource::Live,
-                    SlashCommandDispatchSource::Queued => GoalObjectiveValidationSource::Queued,
-                };
-                if !self.goal_objective_is_allowed(objective, validation_source) {
-                    return;
-                }
                 let Some(thread_id) = self.thread_id else {
                     if source == SlashCommandDispatchSource::Live {
                         self.queue_user_message_with_options(
@@ -792,7 +779,7 @@ impl ChatWidget {
                             },
                             QueuedInputAction::ParseSlash,
                         );
-                        self.bottom_pane.drain_pending_submission_state();
+                        self.clear_live_goal_submission();
                     } else {
                         self.add_info_message(
                             GOAL_USAGE.to_string(),
@@ -808,7 +795,7 @@ impl ChatWidget {
                 });
                 self.append_message_history_entry(format!("/goal {trimmed}"));
                 if source == SlashCommandDispatchSource::Live {
-                    self.bottom_pane.drain_pending_submission_state();
+                    self.clear_live_goal_submission();
                 }
             }
             SlashCommand::Side | SlashCommand::Btw if !trimmed.is_empty() => {
@@ -945,11 +932,6 @@ impl ChatWidget {
             rest_offset + leading_trimmed,
             &text_elements,
         );
-        if cmd == SlashCommand::Goal
-            && !self.goal_objective_is_allowed(trimmed_rest, GoalObjectiveValidationSource::Queued)
-        {
-            return QueueDrain::Continue;
-        }
         self.dispatch_prepared_command_with_args(
             cmd,
             PreparedSlashCommandArgs {