fix(cli-resolvers): negative-result caching, SIGKILL on probes, restored VITEST hermeticity, wired not-found diagnostics

Post-merge follow-ups for PR #329 (shared CLI executable resolution):

- Negative-cache resolution misses with a doubling backoff (1min -> 5min
  cap, cliResolveRetryDelayMs, mirroring claudeVersionRetryDelayMs): the
  shared resolver cached success only, so a missing CLI re-ran the whole
  chain - ending in a synchronous interactive login-shell spawn bounded by
  the 5s EXEC_TIMEOUT_MS - on every /api/<cli>/status request and Run
  attempt, stalling the event loop each time, forever. Success still caches
  for the process lifetime, so an installed CLI is picked up within minutes
  without a restart. Tests drive the backoff via an injectable clock
  (createCliExecutableResolver `now` option, threaded through the
  createPiResolverForTest / createAntigravityResolverForTest wrappers).

- Pass killSignal: 'SIGKILL' on the resolver's login-shell spawn and on the
  pi/claude --version probes: execFileSync's timeout only SENDS the kill
  signal and then keeps waiting for the child to exit, and interactive bash
  ignores SIGTERM, so a login shell stuck in a blocking .bash_profile
  survived the timeout and blocked the server permanently.

- Restore test hermeticity (PR #329 deleted pi's VITEST guards, and one
  test pinned the deletion): under vitest the production resolver host now
  replaces un-injected IO primitives with inert stubs - no real PATH
  scanning, no login-shell spawns - and probePiVersion never executes a
  `pi` candidate again (`pi` is a generic binary name, so route tests
  hitting /api/pi/status executed whatever binary the machine carried).
  Tests opt in through the runCommand/isExecutableFile injection hooks or
  allowRealIoUnderVitest for real-filesystem fixtures. The deletion-pinning
  test is replaced by behavioral pins, including a real-executable fixture
  in the new test/pi-cli-resolver.test.ts that fails loudly if the pi gate
  is ever removed again.

- Wire the six get*NotFoundMessage() exports (previously dead) into their
  intended call sites: the createSession throws in tmux-manager and the
  availability gates on POST /api/sessions and POST /api/quick-start in
  session-routes, replacing a third hardcoded copy of the text. A not-found
  error now names where resolution looked (server PATH, login shell,
  checked directories). npm run knip no longer reports any unused export
  from the resolver modules.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/utils/pi-cli-resolver.ts

@@ -56,13 +56,25 @@ const PI_NOT_FOUND = 'Pi CLI not found. Install with: npm install -g --ignore-sc
  * looks like the coding agent. Returns null for anything else — a missing
  * binary, a non-zero exit, a hang (timeout), or output that is not semver-shaped
  * (which is how an unrelated `pi` on PATH gets rejected).
+ *
+ * Never runs under vitest: the suites must stay hermetic and must not depend on
+ * whether the dev box happens to have pi installed — and since `pi` is a short
+ * GENERIC name, this probe would EXECUTE whatever binary of that name the
+ * machine carries. The shared resolver host is already inert under vitest, so
+ * this gate is defense in depth for any opted-in host that still carries the
+ * default probe; tests drive resolution via `createPiResolverForTest`, whose
+ * injected probe bypasses it. Pinned by test/pi-cli-resolver.test.ts.
  */
 function probePiVersion(binPath: string): string | null {
+  if (process.env.VITEST) return null;
   try {
     const out = execFileSync(binPath, ['--version'], {
       encoding: 'utf-8',
       timeout: EXEC_TIMEOUT_MS,
       stdio: ['ignore', 'pipe', 'ignore'],
+      // A stuck or hostile `pi` that ignores SIGTERM would survive the timeout
+      // and block the server (execFileSync keeps waiting after the signal).
+      killSignal: 'SIGKILL',
     }).trim();
     // Upstream prints a bare version today; tolerate a `pi 0.84.1` style prefix too.
     const candidate = PI_VERSION_REGEX.exec(out)?.[1];
@@ -76,7 +88,7 @@ function probePiVersion(binPath: string): string | null {
 
 type PiVersionProbe = (binPath: string) => string | null;
 
-function createPiResolver(host?: CliResolverHost, versionProbe: PiVersionProbe = probePiVersion) {
+function createPiResolver(host?: CliResolverHost, versionProbe: PiVersionProbe = probePiVersion, now?: () => number) {
   return createCliExecutableResolver<string>(
     {
       binary: 'pi',
@@ -85,14 +97,19 @@ function createPiResolver(host?: CliResolverHost, versionProbe: PiVersionProbe =
         const version = versionProbe(binPath);
         return version ? { accepted: true, metadata: version } : { accepted: false };
       },
+      now,
     },
     host
   );
 }
 
-/** Creates an isolated Pi wrapper around an injected host and version probe. */
-export function createPiResolverForTest(host: CliResolverHost, versionProbe: PiVersionProbe) {
-  return createPiResolver(host, versionProbe);
+/**
+ * Creates an isolated Pi wrapper around an injected host, version probe and
+ * clock. Omitting `versionProbe` keeps the ambient (VITEST-gated) probe, which
+ * is exactly what the hermeticity test exercises.
+ */
+export function createPiResolverForTest(host: CliResolverHost, versionProbe?: PiVersionProbe, now?: () => number) {
+  return createPiResolver(host, versionProbe ?? probePiVersion, now);
 }
 
 const piResolver = createPiResolver();