[codex] Fix runtime-store session checkpointing

[lukacf]

Summary

• remove the runtime_store gate from StoreCheckpointer so runtime-backed sessions still checkpoint changed turns into SessionStore
• clarify that RuntimeStore remains the runtime boundary authority while StoreCheckpointer maintains the compatibility/session projection
• update and add regression coverage for runtime-store-backed checkpointer installation, write behavior, and cancellation behavior
• ensure meerkat-session changes fan out to SDK, minimal, feature, and audit BuildBuddy lanes instead of skipping consumer coverage

Root Cause

0.6.17 installed StoreCheckpointer for every persistent session but set enabled: self.runtime_store.is_none(). MobKit's UnifiedRuntime always installs a runtime_store, so consumers that provide a SessionStore through MobKit stopped receiving per-turn SessionStore saves.

The first CI graph for this PR also showed the same class of under-coverage at the lane detector: meerkat-session/* changes were not triggering SDK/minimal/feature/security lanes, which is wrong for a session persistence regression visible through MobKit/SDK consumers.

Validation

• ./scripts/repo-cargo fmt --check
• ./scripts/repo-cargo test -p meerkat-session --features session-store store_checkpointer -- --nocapture
• ./scripts/repo-cargo test -p meerkat-session --features session-store service_checkpointer -- --nocapture
• ./scripts/repo-cargo test -p meerkat-session --features session-store
• printf 'meerkat-session/src/persistent.rs\n' | scripts/buildbuddy-edge-changes --paths-from-stdin
• make buildbuddy-doctor
• pre-push gates: cargo fmt --check, changed-crate clippy, machine codegen + verify, generated-header audit, bridge invariant, Bazel BUILD/MODULE freshness, workspace deterministic gate

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Defer runtime-backed projection writes until commit succeeds

When this checkpointer is installed for runtime-backed sessions, AgentRunner::checkpoint_current_session runs immediately after a successful LLM/tool loop, but before start_turn_with_machine_protocol commits the runtime terminal receipt and persists the authoritative runtime snapshot. If that later runtime commit fails, the service discards the live session and returns an error, while the newly enabled checkpointer has already written the uncommitted turn into SessionStore, so compatibility consumers can observe a transcript that the runtime boundary rejected. Please keep runtime-backed projection writes behind the runtime commit path (or otherwise roll them back on commit failure) rather than writing from the pre-commit agent checkpointer.

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