fix(local-agents): don't 500 /detect when a version-probe spawn is denied

detectOne() ran the `--version` probe via execFile INSIDE a `new Promise`
executor but only handled the async error path. execFile can throw
SYNCHRONOUSLY (e.g. `spawn EPERM` when child-process creation is restricted),
and a synchronous throw in a Promise executor REJECTS the promise. Because
detectLocalAgents() used Promise.all, one agent's spawn failure rejected the
whole batch → GET /api/agents/local/detect returned 500 → the Settings "Local
Agents" card silently rendered "0/0 installed" even though all CLIs were on PATH.

- detectOne: wrap execFile in try/catch. The binary was already found on PATH
  (resolveBin returned a path) so it IS installed; on a sync throw we resolve
  installed:true with version '?' and filesystem-based auth, never rejecting.
- detectLocalAgents: Promise.allSettled instead of Promise.all, so one agent's
  failure degrades to "not installed" for that agent only, never the batch.

Verified: /detect returns 200 with claude/codex/hermes installed+ready.
Typecheck clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/agent/local-agents.ts

@@ -273,6 +273,17 @@ function detectOne(spec: AgentSpec): Promise<AgentDetection> {
       resolve({ ...base, installed: false, authed: false, ready: false });
       return;
     }
+    // Found on PATH ⇒ installed. Used when the version probe can't RUN (a synchronous spawn
+    // throw like EPERM, or any async error that isn't ENOENT): the binary exists, we just
+    // couldn't read its version. Auth is a filesystem/keychain check, so it still works.
+    const installedNoVersion = () => {
+      const auth = authState(spec);
+      resolve({
+        ...base, installed: true, path: resolved, version: '?',
+        authed: auth.authed, authMethod: auth.method, ready: auth.authed,
+      });
+    };
+    try {
       // Launch the RESOLVED path (a .cmd shim needs a shell; a real .exe does not).
       execFile(resolved, spec.versionArgs, { timeout: 8000, shell: needsShell(resolved) }, (err, stdout) => {
         if (err && (err as NodeJS.ErrnoException).code === 'ENOENT') {
@@ -283,15 +294,17 @@ function detectOne(spec: AgentSpec): Promise<AgentDetection> {
         const version = spec.parseVersion(String(stdout || ''));
         const auth = authState(spec);
         resolve({
-        ...base,
+          ...base, installed: true, path: resolved, version,
-        installed: true,
+          authed: auth.authed, authMethod: auth.method, ready: auth.authed,
-        path: resolved,
-        version,
-        authed: auth.authed,
-        authMethod: auth.method,
-        ready: auth.authed,
         });
       });
+    } catch {
+      // execFile can throw SYNCHRONOUSLY (e.g. `spawn EPERM` when child-process creation is
+      // restricted). Inside this Promise executor a throw would REJECT the promise, which fails
+      // the Promise.allSettled sibling below and — pre-fix, under Promise.all — 500'd the whole
+      // /api/agents/local/detect endpoint, blanking the Settings panel to "0/0 installed".
+      installedNoVersion();
+    }
   });
 }
 
@@ -301,7 +314,17 @@ export async function detectLocalAgents(): Promise<AgentDetection[]> {
   // local-agent auto-detection) so key-required / fail-closed paths can be exercised
   // deterministically — used by scripts/arsenal-smoke.mjs for a reproducible run.
   if (/^(1|true|yes|on)$/i.test((process.env.T3MP3ST_DISABLE_LOCAL_AGENTS || '').trim())) return [];
-  return Promise.all(SPECS.map(detectOne));
+  // allSettled, NOT all: a single agent's detection failure must degrade to "not installed"
+  // for that one agent, never reject the whole batch (which 500'd /detect and blanked the UI).
+  const settled = await Promise.allSettled(SPECS.map(detectOne));
+  return settled.map((r, i) => {
+    if (r.status === 'fulfilled') return r.value;
+    const s = SPECS[i];
+    return {
+      id: s.id, label: s.label, vendor: s.vendor, bin: s.bin, blurb: s.blurb,
+      invokeHint: s.invokeHint, installed: false, authed: false, ready: false,
+    };
+  });
 }
 
 export interface AgentRunResult {