fix(hooks): use shared renameWithRetry in writeWarnState (ecc-context-monitor)

Mirror the previous commit's Windows-EPERM retry on the companion
`writeWarnState` in `scripts/hooks/ecc-context-monitor.js`. Same
race: two PostToolUse subprocesses writing concurrent debounce
state racing on `MoveFileExW`, target-in-use throwing EPERM on
Windows even though each writer's tmp path is now unique.

Implementation: import `renameWithRetry` from `scripts/lib/session-bridge.js`
(exported in the previous commit) instead of duplicating the helper.
The retry policy, backoff schedule, and main-thread `Atomics.wait`
strategy stay identical to `writeBridgeAtomic`.

Three writers in the repo now share the same atomic-write contract:
- `writeBridgeAtomic` (scripts/lib/session-bridge.js) — round 1 +
  this round's retry
- `writeWarnState` (this file) — round 1 + this round's retry via shared helper
- `writeCostWarningIfChanged` (scripts/hooks/ecc-metrics-bridge.js) —
  out of scope for this PR (already uses unique tmp suffix; a future
  consolidation could move it to the shared helper too).

Local: `yarn test` green, `yarn lint` clean. The companion test
suite for `ecc-context-monitor.js` does not currently exercise
concurrent `writeWarnState` writes, but the helper it now uses is
covered by the `tests/lib/session-bridge.test.js` concurrent-write
regression added in round 1's last commit.