Migrate inherit sandbox state to per-tool-call metadata

[t-kalinowski]

Summary

• migrate --sandbox inherit from the obsolete async update flow to per-tool-call _meta["codex/sandbox-state-meta"]
• keep inherit mode fail-closed when a call needs inherited sandbox state but does not provide usable metadata
• preserve local-only follow-ups while staging current metadata before session-end respawns
• document the request-scoped metadata contract and refresh regression coverage around metadata-driven sandbox selection

Why

Current Codex supplies inherited sandbox state on each tool call instead of through the old async update channel. The old contract no longer matched the client, which risked running a tool call without the sandbox state that actually applied to that call.

Public-facing changes

• --sandbox inherit now expects per-tool-call codex/sandbox-state-meta metadata on calls that spawn, respawn, or reset an inherited worker
• missing or malformed metadata still fails closed when current sandbox state is required
• local poll-only paths can still return existing output/status without treating optional malformed metadata as fatal
• explicit non-inherit sandbox modes remain authoritative and ignore Codex metadata
• docs now describe the request-scoped metadata contract instead of the obsolete async update protocol

Internal-only changes

• remove the old async sandbox update listener and startup settle path
• thread request-scoped sandbox metadata through server and worker follow-up plumbing
• ensure session-end resets and interrupt/restart tail paths do not respawn under stale inherited state
• rebuild inherit-mode regression coverage around the current Codex contract, including platform-specific test gating for Unix-only cases

Diff composition

Measured against origin/main, this PR is 5,300 insertions and 1,165 deletions across 46 files. Most of that is regression coverage rather than runtime code:

• runtime code in src/: +1,048/-343 (21.5% of churn)
• tests in tests/: +3,464/-726 (64.8% of churn)
• inline tests inside src/: +648/-77 (11.2% of churn)
• docs and README: +140/-19 (2.5% of churn)

The largest contributors are tests/sandbox_state_updates.rs, tests/codex_approvals_tui.rs, and inline tests in src/worker_process.rs.

Verification

• cargo check
• cargo build
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• cargo +nightly fmt --all -- --check
• codex review --base origin/main
• GitHub Actions CI for PR Migrate inherit sandbox state to per-tool-call metadata #42