[codex] Adopt global instructions contributors

[anp-oai]

Why

With the extension API, CODEX_HOME contributor, and durable snapshot semantics in place, core can stop owning global instruction loading through Config. Completing the migration makes freshness points explicit, lets hosts choose their contributor, and prevents history-sharing thread creation from unexpectedly re-resolving instructions.

What changed

• Resolves GlobalInstructionsContributor for fresh-history creation and full-context rebuilding.
• Does not resolve during ordinary turns, cold or running resume, user or extension forks, or history-sharing subagents.
• Retains the previous snapshot and emits EventMsg::Warning when a compaction refresh fails.
• Surfaces successful contributor warnings through startup and runtime warning reporting.
• Registers CodexHomeInstructionsContributor in app-server, MCP server, prompt-debug, and the thread-manager sample.
• Loads global instructions even when no execution environment is selected.
• Updates stateful integration tests for freshness, failure, compaction, resume, fork, and source reporting semantics.
• Removes guardian reuse-key handling tied to configured user instructions.

Breaking changes

• Removes Config.user_instructions.
• Removes the public LoadedAgentsMd configuration and testing escape hatch.
• Embedders that want CODEX_HOME instructions must install CodexHomeInstructionsContributor in their extension registry.

Stack

1. [codex] Characterize global instruction lifecycle #26830: lifecycle characterization tests
2. [codex] Add global instructions contributor API #26831: extension API primitives
3. [codex] Add CODEX_HOME instructions contributor #26832: CODEX_HOME contributor crate
4. [codex] Persist structured instruction snapshots #26833: structured snapshot and history machinery
5. [codex] Adopt global instructions contributors #26834: contributor adoption and compatibility removal

Test plan

• just test -p codex-extension-api
• just test -p codex-home
• Core global instruction lifecycle, composition, reconstruction, fork, and compaction tests
• Local, legacy remote, and remote-v2 compaction parity
• App-server start and fork instructionSources tests
• MCP and prompt-debug integration tests
• just test -p codex-protocol -p codex-state -p codex-rollout
• just bazel-lock-check