Make plugin guidance react to environment readiness (#30223)

## Why

Generic plugin guidance is currently emitted only with initial context
from host plugin state. An executor-selected plugin can become available
later in the same turn, making its skills and tools usable without ever
telling the model how plugin capabilities should be used.

## What

- project every ready selected plugin package, including skill-only
plugins
- carry plugin availability with the exact MCP runtime projection while
preserving MCP manager reuse when servers and connectors are unchanged
- move generic plugin guidance from the static initial-context path into
persisted World State
- recognize legacy and retained plugin fragments so resume and
compaction do not duplicate guidance

## Testing

- `just test -p codex-mcp-extension`
- `just test -p codex-core plugins_instructions`
- `just test -p codex-core
plugin_availability_change_reuses_the_mcp_manager`
- `just test -p codex-app-server --test all selected_capabilit`

codex-rs/core/src/context_manager/history_tests.rs

@@ -22,6 +22,7 @@ use codex_protocol::openai_models::InputModality;
 use codex_protocol::openai_models::default_input_modalities;
 use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::InterAgentCommunication;
+use codex_protocol::protocol::PLUGINS_INSTRUCTIONS_OPEN_TAG;
 use codex_protocol::protocol::SandboxPolicy;
 use codex_protocol::protocol::TurnContextItem;
 use codex_utils_absolute_path::AbsolutePathBuf;
@@ -1021,6 +1022,9 @@ fn drop_last_n_user_turns_trims_context_updates_above_rolled_back_turn() {
         user_input_text_msg("turn 1 user"),
         assistant_msg("turn 1 assistant"),
         developer_msg("Generated images are saved to /tmp as /tmp/image-1.png by default."),
+        developer_msg(&format!(
+            "{PLUGINS_INSTRUCTIONS_OPEN_TAG}\nROLLED_BACK_PLUGIN_INSTRUCTIONS"
+        )),
         developer_msg("<collaboration_mode>ROLLED_BACK_DEV_INSTRUCTIONS</collaboration_mode>"),
         developer_msg("<multi_agent_mode>ROLLED_BACK_MULTI_AGENT_MODE</multi_agent_mode>"),
         user_input_text_msg(