fix(doctor): Diagnostics landing fixes (#536)

- The doctor now judges candidates like the run mode's resolver: the PATH
  hit, then each search dir, each one version-checked on its own and
  skipped on a mismatch (a wrong `pi`/`grok` on the PATH no longer hides
  the real one in a search dir). A search-dir candidate must be an
  absolute path to an executable regular file, so a relative dir or a
  file without the x bit reads as missing, as it does in the Run menu.
  `isExecutableRegularFile` is exported from cli-executable-resolver.ts
  and reused rather than copied.
- Every doctor probe passes killSignal: 'SIGKILL'; a --version that
  ignores SIGTERM held the probe for its full runtime (15 s vs 5 s
  measured with a TERM-trapping script).
- README no longer claims parity with the Run menu or nvm prefixes.
- The Diagnostics panel marks a missing optional tool with ○, a missing
  required one with ✗, as the terminal doctor does.
- expandSearchDir names its twin, expandHome() in cli-resolver.ts.
- test/doctor-cli-json.test.ts is hermetic: temp HOME, a PATH of only
  `which` and `node`, and a clis.json that drops the registry's absolute
  search dirs, so it never runs the machine's installed agent CLIs.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/config/dependency-registry.ts

@@ -41,7 +41,11 @@ export interface PathResolver {
   searchDirs?: string[];
 }
 
-/** Expand a leading `~` (the only form registry `searchDirs` use). */
+/**
+ * Expand a leading `~` (the only form registry `searchDirs` use). Twin of `expandHome()` in
+ * src/utils/cli-resolver.ts, copied rather than imported because importing it from config/
+ * would pull in the whole resolver chain; keep the two in step.
+ */
 function expandSearchDir(dir: string): string {
   if (dir === '~') return homedir();
   if (dir.startsWith('~/')) return join(homedir(), dir.slice(2));