[codex] Persist structured instruction snapshots

[anp-oai]

Why

History-sharing threads must retain the instructions that were active when their history was produced. That persistence and inheritance behavior should be established before changing where fresh global instructions are resolved, so resume, forks, truncated subagents, and compaction do not accidentally reload or lose instructions.

What changed

• Adds structured instruction snapshots with global, project, and internal provenance.
• Stores the current snapshot in session state separately from rendered history.
• Persists snapshots in durable turn-context items, including source paths.
• Restores snapshots during resume and fork reconstruction with rollback-aware behavior.
• Carries metadata-only snapshots through truncated-history subagent forks without claiming a full context baseline.
• Refreshes only the global portion during full-context rebuilding while retaining the existing Config.user_instructions source for compatibility.
• Leaves legacy histories without a structured snapshot empty until a later full-context rebuild.
• Removes the redundant rendered instruction copy from TurnContext.
• Bounds persisted and model-visible instruction content.

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

• Core instruction composition and bounding tests
• Rollout reconstruction and rollback tests
• Cold resume and fork source inheritance tests
• Truncated-history subagent fork coverage
• Local, legacy remote, and remote-v2 compaction paths
• App-server fork instructionSources coverage