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>

test/antigravity-cli-resolver.test.ts

@@ -5,7 +5,11 @@ import { homedir } from 'node:os';
 import { join } from 'node:path';
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 import { createAntigravityResolverForTest, isAntigravityAvailable } from '../src/utils/antigravity-cli-resolver.js';
-import type { CliResolution, CliResolverHost } from '../src/utils/cli-executable-resolver.js';
+import {
+  cliResolveRetryDelayMs,
+  type CliResolution,
+  type CliResolverHost,
+} from '../src/utils/cli-executable-resolver.js';
 
 const availabilityResolution = vi.hoisted(() => ({ current: null as CliResolution | null }));
 
@@ -84,13 +88,19 @@ describe('Antigravity CLI resolver', () => {
     expect(resolver.resolve()).toBeNull();
   });
 
-  it('retries a failed lookup and caches the first successful login-shell discovery', () => {
+  it('retries a failed lookup after the backoff and caches the first successful login-shell discovery', () => {
     const binaryPath = '/late-login-shell/bin/agy';
+    let now = 0;
     const resolver = createAntigravityResolverForTest(
-      createHost({ loginShellResults: [null, binaryPath], existingPaths: [binaryPath] })
+      createHost({ loginShellResults: [null, binaryPath], existingPaths: [binaryPath] }),
+      () => now
     );
 
     expect(resolver.resolve()).toBeNull();
+    // A miss is negative-cached: within the backoff window nothing re-runs the
+    // chain (its login-shell tail is a synchronous bounded spawn in production).
+    expect(resolver.resolve()).toBeNull();
+    now = cliResolveRetryDelayMs(1);
     expect(resolver.resolve()?.binaryPath).toBe(binaryPath);
     expect(resolver.resolve()?.binaryPath).toBe(binaryPath);
   });