[codex] add workspace snapshot turn metadata

[kliu128]

Summary

• add the under-development git_workspace_snapshot feature flag
• attach workspace_snapshot_commit_hash to async turn metadata when the feature is enabled
• compute the snapshot hash from a temporary alternate index and a deterministic synthetic commit, always including untracked files
• add unit and integration coverage plus the matching config schema update
• remove the stray top-level plan.md from the worktree branch

Why

Codex currently records HEAD and has_changes, but it does not expose a stable identifier for the actual dirty workspace state. This change adds that identifier without moving the work onto the session-start path.

User Impact

When git_workspace_snapshot is enabled, turn metadata includes a workspace_snapshot_commit_hash that equals HEAD for a clean repo and a synthetic snapshot commit hash for a dirty repo.

Validation

• cargo test -p codex-features
• RUSTY_V8_ARCHIVE=/Users/kevin.liu/code/codex/codex-rs/target/debug/gn_out/obj/librusty_v8.a cargo test -p codex-core --lib workspace_snapshot_commit
• RUSTY_V8_ARCHIVE=/Users/kevin.liu/code/codex/codex-rs/target/debug/gn_out/obj/librusty_v8.a cargo test -p codex-core --test responses_headers responses_stream_includes_workspace_snapshot_commit_hash_when_feature_enabled -- --exact
• RUSTY_V8_ARCHIVE=/Users/kevin.liu/code/codex/codex-rs/target/debug/gn_out/obj/librusty_v8.a cargo test -p codex-core config::schema::tests::config_schema_matches_fixture -- --exact
• RUSTY_V8_ARCHIVE=/Users/kevin.liu/code/codex/codex-rs/target/debug/gn_out/obj/librusty_v8.a cargo test -p codex-core
  • this full package run still showed unrelated existing failures in arc_monitor, plugin sync, and js_repl tests; the snapshot-related tests passed
• RUSTY_V8_ARCHIVE=/Users/kevin.liu/code/codex/codex-rs/target/debug/gn_out/obj/librusty_v8.a just fix -p codex-core
• just fmt

Notes

• just argument-comment-lint could not run in this checkout because tools/argument-comment-lint/run-prebuilt-linter.sh is missing.

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[kliu128]

I have read the CLA Document and I hereby sign the CLA