feat(agent): report cloud rtk token savings

[tatoalo]

Problem

Cloud runs can use RTK to compress verbose command output, but current production telemetry only compares total run tokens between rollout cohorts. That comparison is affected by workload and run length, so it cannot measure the command-output savings directly.

Changes

• Read RTK's cumulative gain summary at terminal cleanup without exposing process secrets to the subprocess.
• Emit a once-only _posthog/rtk_savings event before event ingest closes, including runtime and model context.
• Keep the reporting path best-effort so missing, malformed, or failing RTK telemetry never affects a run.
• Isolate terminal-failure tests from RTK resolution so they continue to assert event ordering deterministically.

[trunk-io]

Merging to main in this repository is managed by Trunk.

• To merge this pull request, check the box to the left or comment /trunk merge below.

After your PR is submitted to the merge queue, this comment will be automatically updated with its status. If the PR fails, failure details will also be posted here

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 9279717.}

[github-actions]

Well-contained additive feature: emits a new RTK savings notification once per run with proper idempotency guard, secure env scrubbing on the subprocess call, timeout, and silent error handling. Good test coverage for all edge cases, no existing code paths changed.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(agent): stabilize rtk savings shutdo..." | Re-trigger Greptile}

New commits pushed (delta classified non_trivial_delta) — stamphog approval dismissed; re-review running automatically.

[github-actions]

Additive feature that emits RTK token savings once per run at session cleanup. The env-scrubbing concern raised in review was fixed (RTK_DB_PATH preserved in allowlist), idempotency is guarded by rtkSavingsAttempted flag, errors are silently swallowed, subprocess has a timeout, and test coverage is thorough.