fix(lib): use unique tmp suffix in writeBridgeAtomic to eliminate ENOENT race

`writeBridgeAtomic` wrote to a fixed `${target}.tmp` path before
calling `renameSync`. When two processes write to the same session
bridge concurrently (e.g. PostToolUse `ecc-metrics-bridge` + the
background `ecc-statusline`, both calling `writeBridgeAtomic(sessionId, ...)`),
the canonical atomic-rename race fires:

  1. Process A: writeFileSync(target.tmp, JSON_A) — tmp file exists.
  2. Process B: writeFileSync(target.tmp, JSON_B) — tmp file overwritten.
  3. Process A: renameSync(target.tmp, target) — succeeds; target = JSON_B
     (A's payload silently corrupted en-route).
  4. Process B: renameSync(target.tmp, target) — throws ENOENT (the
     rename consumed the file).

Every caller in the repo wraps `writeBridgeAtomic` in `try {} catch {}`,
so the ENOENT exception is swallowed and the user-visible symptom is
just "the bridge file occasionally contains the wrong process's
payload" with no diagnostic.

Reproduced before this commit:

  $ # two concurrent writers, each calling writeBridgeAtomic 500 times
  $ # against the same session ID
  [A] errors=244   # 244 ENOENT exceptions swallowed
  [B] errors=248   # ditto

After this commit the same workload reports 0 errors in both
subprocesses: tmp paths no longer collide.

Fix: change `${target}.tmp` to
`${target}.${process.pid}.${crypto.randomBytes(4).toString('hex')}.tmp`,
matching the pattern already used by `writeCostWarningIfChanged` in
`scripts/hooks/ecc-metrics-bridge.js` (commit 9b1d8918). The pid +
4-byte nonce gives each writer process a distinct tmp path, so step 2
above no longer overwrites step 1's payload and step 4 no longer
races step 3.

Also added: on `renameSync` failure, attempt `fs.unlinkSync(tmp)` so
a writer that fails (disk full, permission, parent dir gone) does
not leak its tmp file. The cleanup is best-effort and the original
error is still re-thrown.

**Scope clarification.** This commit closes the atomic-rename
primitive's race only. The *read-modify-write* race in callers —
two writers each read the same bridge state, increment, and write
back, the second clobbering the first — is a separate concern that
needs locking or per-writer logs, and is intentionally out of scope
for this PR. The cost-tracker / metrics-bridge callers tolerate
last-writer-wins on their cumulative aggregates today and this
commit does not change that contract.

The companion `writeWarnState` in `ecc-context-monitor.js` has the
same fixed-suffix pattern and the same race; that fix lands in the
next commit so each can be reviewed against its own diff.

scripts/lib/session-bridge.js

@@ -8,6 +8,7 @@
  * without scanning large JSONL logs on every invocation.
  */
 
+const crypto = require('crypto');
 const fs = require('fs');
 const os = require('os');
 const path = require('path');
@@ -51,15 +52,35 @@ function readBridge(sessionId) {
 }
 
 /**
- * Write bridge data atomically (write .tmp then rename).
+ * Write bridge data atomically (write unique-suffix tmp then rename).
+ *
+ * The tmp path includes `process.pid` plus a random nonce so concurrent
+ * writers (e.g. PostToolUse `ecc-metrics-bridge` and the background
+ * `ecc-statusline`, both writing to the same session bridge) do not
+ * clobber each other's tmp file mid-write. With a fixed `.tmp` suffix
+ * two writers could both call `writeFileSync` against the same path
+ * before either reaches `renameSync`, causing one writer's payload to
+ * silently overwrite the other and the second `renameSync` to throw
+ * ENOENT once the rename consumes the file.
+ *
+ * Same pattern already used by `writeCostWarningIfChanged` in
+ * `scripts/hooks/ecc-metrics-bridge.js` (commit 9b1d8918) for the
+ * cost-warning cache; this commit applies it to the session-bridge
+ * primitive too.
+ *
  * @param {string} sessionId - Already-sanitized session ID
  * @param {object} data
  */
 function writeBridgeAtomic(sessionId, data) {
   const target = getBridgePath(sessionId);
-  const tmp = `${target}.tmp`;
+  const tmp = `${target}.${process.pid}.${crypto.randomBytes(4).toString('hex')}.tmp`;
   fs.writeFileSync(tmp, JSON.stringify(data), 'utf8');
+  try {
     fs.renameSync(tmp, target);
+  } catch (err) {
+    try { fs.unlinkSync(tmp); } catch { /* ignore */ }
+    throw err;
+  }
 }
 
 /**