Add negotiated session bootstrap identity to Codex 0.147

[matthewgapp]

Summary

Adds an optional, experimental, explicitly negotiated app-server capability for installing a one-shot, in-memory bootstrap identity into an idle loaded Codex thread:

• thread/bootstrapIdentity/install with strict parameters and a typed receipt
• exact listener-incarnation/subscription and idle/no-prior-work admission proof
• startup prewarm quiescence and transport-cache invalidation before success
• same-session-only effective instructions for later turn and compaction provider requests
• no persistence, rollout/timeline event, user input, model/tool effect, or child-agent inheritance from installation
• raw bootstrap identity excluded from durable traces, transport caches, and Debug output
• effective token accounting and pre-compaction state published atomically before the receipt

Compatibility and product boundary

This is an optional experimental negotiated capability. It is advertised only to an app-server connection that enabled the experimental API; callers must typed-stop when it is absent.

Stock and backward-compatible Codex remain supported for their existing behavior, including existing read/resume operations. This change does not assume that an ordinary user-installed Codex exposes the new method.

This PR does not bundle, download, select, or install a default Codex binary. It contains no Blob repository integration, makes no E2E-fixture assumption, and performs no local product installation.

The source base is pinned to official Codex 0.147 commit be6e8eac029b183056b7e4402879f15d2c85f61b. The PR head is the reviewed commit f8b6003c966517d8a2a3ac37de650ee22ab2d0d2.

Validation

Focused and proportional proof on the pinned source:

• affected cargo check --tests: passed
• core bootstrap-focused unit tests: 15 passed
• background-terminal admission regression: passed
• normal + Responses-Lite trace/cache/Debug non-leak tests: passed
• completed-response cache fingerprint test: passed
• effective accounting/recompute and maximum-8192-byte first-compaction tests: passed
• full codex-app-server-protocol suite: 286 passed, 1 ignored fixture writer
• serialized bootstrap app-server E2E suite: 6 passed
• affected all-target Clippy: exit 0; only unrelated pre-existing warnings
• formatting and diff checks: passed

Broad harness caveat: an unfiltered app-server library run is not claimed as passing. Its message_processor::...turn_start_jsonrpc_span_parents_core_turn_spans test stack-overflows in this local harness; the same failure was reproduced on the clean pre-repair parent, so the full protocol suite and focused serialized app-server E2E suite above are the reported executable proof.

Non-goals

• no persisted identity or durable schema
• no replacement/retry of an installed identity
• no realtime identity propagation
• no binary distribution or installation
• no Blob-side adoption in this PR