[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/core/src/session/mod.rs

@@ -294,7 +294,6 @@ use crate::SkillLoadOutcome;
 use crate::SkillMetadata;
 use crate::SkillsManager;
 use crate::agents_md::load_project_instructions;
-use crate::context::UserInstructions;
 use crate::exec_policy::ExecPolicyUpdateError;
 use crate::guardian::GuardianReviewSessionManager;
 use crate::mcp::McpManager;
@@ -527,13 +526,9 @@ impl Codex {
         config
             .startup_warnings
             .extend(user_instruction_provider_warnings);
-        // TODO(anp) assemble instructions from multiple environments
-        let primary_environment = environment_selections.primary_environment();
-        let primary_fs = primary_environment
-            .as_ref()
-            .map(|environment| environment.get_filesystem());
         let loaded_agents_md =
-            load_project_instructions(&mut config, user_instructions, primary_fs.as_deref()).await;
+            load_project_instructions(&mut config, user_instructions, &environment_selections)
+                .await;
 
         let exec_policy = if crate::guardian::is_guardian_reviewer_source(&session_source) {
             // Guardian review should rely on the built-in shell safety checks,
@@ -2978,14 +2973,7 @@ impl Session {
             }
         }
         if let Some(user_instructions) = turn_context.user_instructions.as_deref() {
-            contextual_user_sections.push(
-                UserInstructions {
-                    text: user_instructions.to_string(),
-                    #[allow(deprecated)]
-                    directory: turn_context.cwd.to_string_lossy().into_owned(),
-                }
-                .render(),
-            );
+            contextual_user_sections.push(user_instructions.to_string());
         }
         // This is full-context metadata. Steady-state context diffs should not re-emit it.
         if turn_context.features.enabled(Feature::TokenBudget)

codex-rs/core/src/session/turn_context.rs

@@ -553,7 +553,7 @@ impl Session {
             user_instructions: session_configuration
                 .loaded_agents_md
                 .as_ref()
-                .map(LoadedAgentsMd::text),
+                .map(LoadedAgentsMd::render),
             collaboration_mode: session_configuration.collaboration_mode.clone(),
             multi_agent_version,
             personality: session_configuration.personality,