[codex] Load AGENTS.md from all bound environments (#27696)

## Why

We already have the machinery to support multiple environments on a
single thread, but we only show the model the contents of `AGENTS.md`
files in the primary environment.

We should show the model all of the relevant project instructions when
we know there's more than one environment.

## Known Gaps

As discussed in the RFC, this implementation:

1. doesn't handle environments being added/removed to/from the thread
after its creation
2. it doesn't enforce an aggregate context budget across environments,
and instead applies the configured project maximum independently to each
environment

## Implementation

- Discover project instructions in environment order with an independent
byte budget per environment and preserve source provenance/order.
- Keep the legacy fragment byte-for-byte when exactly one environment
contributes project instructions; use environment-labeled sections when
two or more environments contribute.
- Freeze the complete rendered fragment in `LoadedAgentsMd`, insert it
directly into requests, and recognize both layouts in contextual and
memory filtering.
- Add exact rendering, independent-budget, source-order,
creation-snapshot, and consumer coverage without changing app-server
schemas.

codex-rs/memories/write/src/phase1.rs

@@ -460,7 +460,7 @@ mod job {
             return false;
         };
 
-        matches_marked_fragment(text, "# AGENTS.md instructions for ", "</INSTRUCTIONS>")
+        matches_marked_fragment(text, "# AGENTS.md instructions", "</INSTRUCTIONS>")
             || matches_marked_fragment(text, "<skill>", "</skill>")
     }
 
@@ -487,6 +487,10 @@ mod job {
                     "# AGENTS.md instructions for /tmp\n\n<INSTRUCTIONS>\nbody\n</INSTRUCTIONS>",
                     true,
                 ),
+                (
+                    "# AGENTS.md instructions\n\n<INSTRUCTIONS>\nbody\n</INSTRUCTIONS>",
+                    true,
+                ),
                 (
                     "<skill>\n<name>demo</name>\n<path>skills/demo/SKILL.md</path>\nbody\n</skill>",
                     true,
@@ -665,6 +669,10 @@ mod tests {
                         "# AGENTS.md instructions for /tmp\n\n<INSTRUCTIONS>\nbody\n</INSTRUCTIONS>"
                             .to_string(),
                 },
+                ContentItem::InputText {
+                    text: "# AGENTS.md instructions\n\n<INSTRUCTIONS>\nbody\n</INSTRUCTIONS>"
+                        .to_string(),
+                },
                 ContentItem::InputText {
                     text: "<environment_context>\n<cwd>/tmp</cwd>\n</environment_context>"
                         .to_string(),