fix(hooks): use unique tmp suffix in writeWarnState (ecc-context-monitor)

Mirror the previous commit's `writeBridgeAtomic` fix on the
companion `writeWarnState` in `ecc-context-monitor.js`. Same shape:
fixed `${target}.tmp` → `${target}.${process.pid}.${randomNonce}.tmp`,
plus best-effort cleanup of the tmp file on `renameSync` failure
(throws original error after cleanup).

`writeWarnState` debounces the context-monitor's threshold alarms
(`COST_NOTICE_USD`, `COST_WARNING_USD`, `COST_CRITICAL_USD`, plus the
context-remaining and loop-detection ones). Without unique suffixes,
two PostToolUse subprocesses racing on the warn-state file produce
either a corrupted JSON debounce-state on disk or an ENOENT throw
that the hook catches and swallows — either way the next warn-state
read returns the default `{callsSinceWarn: 0, lastSeverity: null}`
and the threshold alarms re-fire or stop firing erratically. Users
see warning messages flicker or vanish; debounce no longer works.

Three call sites in this repo now share the same atomic-write
contract:
- `writeBridgeAtomic` (scripts/lib/session-bridge.js) — primary
- `writeCostWarningIfChanged` (scripts/hooks/ecc-metrics-bridge.js) — cost cache
- `writeWarnState` (this file) — debounce state

`yarn lint` clean. Regression test covering both `writeBridgeAtomic`
and `writeWarnState` under concurrent load lands in the next commit.

scripts/hooks/ecc-context-monitor.js

@@ -9,6 +9,7 @@
 
 'use strict';
 
+const crypto = require('crypto');
 const fs = require('fs');
 const os = require('os');
 const path = require('path');
@@ -61,15 +62,30 @@ function readWarnState(sessionId) {
 }
 
 /**
- * Write debounce state.
+ * Write debounce state atomically (unique-suffix tmp then rename).
+ *
+ * The tmp path includes `process.pid` plus a random nonce so concurrent
+ * PostToolUse subprocesses writing to the same session's warn-state
+ * file do not clobber each other's tmp mid-write. Without the unique
+ * suffix, two writers race over a shared `${target}.tmp` and produce
+ * either a corrupted payload or an ENOENT throw on the second rename.
+ *
+ * Same pattern as `writeBridgeAtomic` in `scripts/lib/session-bridge.js`
+ * and `writeCostWarningIfChanged` in `scripts/hooks/ecc-metrics-bridge.js`.
+ *
  * @param {string} sessionId
  * @param {object} state
  */
 function writeWarnState(sessionId, state) {
   const target = getWarnPath(sessionId);
-  const tmp = `${target}.tmp`;
+  const tmp = `${target}.${process.pid}.${crypto.randomBytes(4).toString('hex')}.tmp`;
   fs.writeFileSync(tmp, JSON.stringify(state), 'utf8');
-  fs.renameSync(tmp, target);
+  try {
+    fs.renameSync(tmp, target);
+  } catch (err) {
+    try { fs.unlinkSync(tmp); } catch { /* ignore */ }
+    throw err;
+  }
 }
 
 /**