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/tmux-manager.ts

@@ -75,11 +75,17 @@ import {
   SAFE_PATH_PATTERN,
   findClaudeDir,
   getClaudeCliVersion,
+  getClaudeNotFoundMessage,
   resolveOpenCodeDir,
+  getOpenCodeNotFoundMessage,
   resolveCodexDir,
+  getCodexNotFoundMessage,
   resolveGeminiDir,
+  getGeminiNotFoundMessage,
   resolveAntigravityDir,
+  getAntigravityNotFoundMessage,
   resolvePiDir,
+  getPiNotFoundMessage,
   resolveLocalShell,
   loginShellArgs,
 } from './utils/index.js';
@@ -1853,29 +1859,28 @@ export class TmuxManager extends EventEmitter implements TerminalMultiplexer {
       return session;
     }
 
-    // Resolve CLI binary directory based on mode
+    // Resolve CLI binary directory based on mode. The not-found messages come
+    // from the resolvers (formatCliNotFoundMessage) so the error names WHERE it
+    // looked — server PATH, login shell, checked directories — instead of just
+    // asserting the CLI is missing (the classic systemd/launchd PATH trap).
     const { pathExport, dir: cliDir } = this.buildPathExport(mode);
     if (mode === 'claude' && !cliDir) {
-      throw new Error('Claude CLI not found. Install it with: curl -fsSL https://claude.ai/install.sh | bash');
+      throw new Error(getClaudeNotFoundMessage());
     }
     if (mode === 'opencode' && !cliDir) {
-      throw new Error('OpenCode CLI not found. Install with: curl -fsSL https://opencode.ai/install | bash');
+      throw new Error(getOpenCodeNotFoundMessage());
     }
     if (mode === 'codex' && !cliDir) {
-      throw new Error('Codex CLI not found. Install with: npm install -g @openai/codex');
+      throw new Error(getCodexNotFoundMessage());
     }
     if (mode === 'gemini' && !cliDir) {
-      throw new Error('Gemini CLI not found. Install with: npm install -g @google/gemini-cli');
+      throw new Error(getGeminiNotFoundMessage());
     }
     if (mode === 'antigravity' && !cliDir) {
-      throw new Error(
-        'Antigravity CLI not found. Install with: curl -fsSL https://antigravity.google/cli/install.sh | bash'
-      );
+      throw new Error(getAntigravityNotFoundMessage());
     }
     if (mode === 'pi' && !cliDir) {
-      throw new Error(
-        'Pi CLI not found. Install with: npm install -g --ignore-scripts @earendil-works/pi-coding-agent'
-      );
+      throw new Error(getPiNotFoundMessage());
     }
 
     const envExportsStr = this.buildEnvExports(sessionId, muxName, mode).join(' && ');