core: reconcile legacy WorldState sections

[sayan-oai]

Why

Older rollouts can retain model-visible context for a WorldState section without having a persisted snapshot for that section. Treating the missing snapshot as definitely absent can duplicate old context or fail to tell the model that it was replaced or removed.

This provides a generic migration path for sections moving into WorldState, beginning with AGENTS.md.

Builds on #29810.

What changed

• distinguish section state that is absent, known from a persisted snapshot, or unknown because matching legacy context remains in history
• let WorldState sections identify their own legacy fragments while ContextManager owns history reconciliation and baseline persistence
• make AGENTS.md emit one conservative replacement or removal update for legacy history, then deduplicate from the newly persisted baseline
• preserve existing environment rendering when persisted section data is missing or malformed

Testing

• just test -p codex-core world_state
• just test -p codex-core cold_resume_invalidates_deleted_legacy_agents_md_once -- --exact

[chatgpt-codex-connector]

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