Two regression tests pin down the previous two commits' atomic-rename
fixes:
1. **concurrent writes don't throw ENOENT or corrupt the file** —
spawns two child Node processes (`tests/__tmp_bridge_writer.js`
created in-test, cleaned up in finally) that each call
`writeBridgeAtomic(sid, …)` 200 times against the same session
ID with independent payloads. Asserts both subprocesses exit 0
(the previous implementation produced ENOENT on roughly 50% of
rename calls, all swallowed by the in-test catch) and the final
bridge file is parseable JSON belonging to one of the two writers
(last-writer-wins is fine; the contract is *no corruption* and
*no rename ENOENT*, not data preservation).
2. **tmp file cleanup on rename failure** — pre-creates a directory
at the target bridge path so `renameSync(tmp, target)` fails,
calls `writeBridgeAtomic`, asserts the call throws AND that no
tmp file with the writer's `pid.<nonce>.tmp` prefix is left
behind in `os.tmpdir()`. The previous code had no cleanup; the
fix's `try/catch + unlinkSync` keeps tmpdir from accumulating
orphan files across repeated rename failures.
The first test deliberately writes independent payloads from each
subprocess so this regression doesn't try to claim a property the
fix doesn't actually deliver (read-modify-write race in the caller
is a separate issue and out of scope per PR body).
Test count: 12 → 14 in `tests/lib/session-bridge.test.js`;
`npm test` green; `npm run lint` clean.
Salvages the useful statusline/context monitor work from stale PR #1504 while preserving the current continuous-learning hook runner wiring.
Adds the metrics bridge, context monitor, statusline script, shared cost/session bridge utilities, and tests. Fixes the reviewed false loop-detection hash collision for non-file tools, avoids default-session cost inflation, sanitizes statusline task lookup, and records hook payload session IDs in cost-tracker.
