[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/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,