v1.13.8: system-prompt prefix stability verify-and-measure

Recon during planning disproved the original v1.13.7 (DB-cache) premise:
buildSystemPrompt already runs over inputs mtime-cached at the file layer
(BOOCHAT.md in system-prompt.ts:25, AGENTS.md global+per-project in
agents.ts:245), and DB scalars are byte-stable until edited. The output
is microsecond pure-string concat with no I/O. Skills aren't in the
prefix; tools live in a separate request body field alpha-sorted by
v1.13.3.

This batch closes the verification gap with instrumentation, not
implementation:

- system-prompt.ts: buildSystemPromptWithFingerprint canonical impl
  computes SHA-256 over the assembled prefix, runs a per-session
  Map<sessionId, lastHash> observer, emits PrefixFingerprint per call
  and PrefixDrift (with field-level changed_inputs) on hash change.
  buildSystemPrompt is now a thin shim returning .prompt.
- agents.ts: getAgentsMtimes accessor — cache-read only, no I/O.
- payload.ts: buildMessagesPayload takes optional log argument; when
  passed, emits prefix-fingerprint (info) + prefix-drift (warn).
- turn.ts + sentinel-summaries.ts: pass ctx.log at 3 production call
  sites; sentinel summaries log too so any drift across cap-hit /
  doom-loop paths surfaces.
- system-prompt.test.ts: 4 new tests (byte-identical, no-drift-on-
  stable, drift-fires-with-changed-inputs, cross-session-no-drift).

194/194 tests pass (was 190).

Smoke: 5 messages in a fresh session produced 7 prefix-fingerprint
logs (extras from buildMessagesPayload being called from sentinel
summary paths), all with identical prefix_hash and prefix_length=2907,
zero prefix-drift. Prefix is byte-stable in steady-state.

Decision: original system_prompt_cache DB table from the roadmap is
permanently dropped. The v1.12.0 mtime caches at the input layer plus
alpha tool ordering at the request body (v1.13.3) already address the
load-bearing cache-stability surfaces. Instrumentation stays so the
claim can be re-verified at any time.

apps/server/src/services/agents.ts

@@ -252,6 +252,22 @@ export function invalidateAgentsCache(projectPath?: string): void {
   }
 }
 
+// v1.13.8: cache-read accessor for the system-prompt prefix-fingerprint log.
+// Returns the AGENTS.md mtimes that getAgentsForProject() observed on its
+// last cache fill for this projectPath. Both fields are null when the cache
+// is cold (e.g. tests, fresh boot before the first inference turn). Does no
+// I/O — a fresh stat would race the cache and isn't what the fingerprint
+// wants anyway (we want what was actually used to resolve the agent).
+export function getAgentsMtimes(projectPath: string): {
+  global: number | null;
+  project: number | null;
+} {
+  const key = projectPath || '__none__';
+  const entry = cache.get(key);
+  if (!entry) return { global: null, project: null };
+  return { global: entry.globalMtime, project: entry.projectMtime };
+}
+
 async function safeStat(path: string): Promise<number | null> {
   try {
     const s = await fs.stat(path);