refactor(#3309): extend planning-snapshot.cts with config/agentInstall/worktreeHealth

Phase 11 of epic #3180 (ADR-3180 §8.2/§8.3/§8.5) foundation. Extends the
already-merged Phase-10 PlanningSnapshot additively with three fields the
upcoming health-diagnostic rule table needs and Phase 10 never required:

- config: {value, scope, exists} — parsed .planning/config.json. `exists`
  distinguishes absent (no diagnostic, non-answer) from present-but-invalid
  (CONFIG_UNREADABLE diagnostic, corruption) — both collapse to scope
  UNREADABLE, so a rule needs the extra bit to tell "not configured yet"
  apart from "config.json is broken."
- agentInstall / worktreeHealth — not .planning/-sourced, wrap the existing
  checkAgentsInstalled/inspectWorktreeHealth owners with the same arguments
  cmdValidateHealth already passes them, so a later migration step reads
  these fields instead of calling the owners itself.

Adds CONFIG_UNREADABLE to src/unusable-input.cts's UNUSABLE_REASON (eighth
#1879 site), mirroring STATE_UNREADABLE's exact shape from Phase 10.

Additive only — the four Phase-10 fields and worstScope/buildPhaseSnapshot
are unchanged; existing tests for them are untouched.

tests/unusable-input.test.cjs

@@ -72,7 +72,7 @@ describe('UNUSABLE_REASON', () => {
     // (enum + call site + this assertion) instead of a silent widening.
     assert.deepStrictEqual(
       Object.keys(UNUSABLE_REASON).sort(),
-      ['FRONTMATTER_UNTERMINATED', 'LAST_ACTIVITY_UNPARSEABLE', 'ROADMAP_UNREADABLE', 'STATE_UNREADABLE'],
+      ['CONFIG_UNREADABLE', 'FRONTMATTER_UNTERMINATED', 'LAST_ACTIVITY_UNPARSEABLE', 'ROADMAP_UNREADABLE', 'STATE_UNREADABLE'],
     );
     assert.strictEqual(UNUSABLE_REASON.FRONTMATTER_UNTERMINATED, 'frontmatter_unterminated');
   });
@@ -134,6 +134,53 @@ describe('STATE_UNREADABLE', () => {
   });
 });
 
+// ─── CONFIG_UNREADABLE: a config.json that exists but could not be read/parsed ─
+
+describe('CONFIG_UNREADABLE', () => {
+  test('a genuinely unreadable config.json produces exactly one diagnostic', () => {
+    _resetUnusableInputWarningsForTests();
+    const emitted = emissionsDuring(() => {
+      const wrote = warnUnusableInput({
+        reason: UNUSABLE_REASON.CONFIG_UNREADABLE,
+        source: '/u/config-unreadable.json',
+      });
+      assert.strictEqual(wrote, true);
+    });
+    assert.strictEqual(emitted, 1);
+  });
+
+  test('the same config.json path reported twice yields one diagnostic', () => {
+    _resetUnusableInputWarningsForTests();
+    const source = '/u/config-unreadable-dedup/config.json';
+    const emitted = emissionsDuring(() => {
+      const first = warnUnusableInput({ reason: UNUSABLE_REASON.CONFIG_UNREADABLE, source });
+      const repeat = warnUnusableInput({ reason: UNUSABLE_REASON.CONFIG_UNREADABLE, source });
+      assert.strictEqual(first, true);
+      assert.strictEqual(repeat, false, 'same (path, cause) must dedup');
+    });
+    assert.strictEqual(emitted, 1);
+  });
+
+  test('two different config.json paths are never suppressed as one', () => {
+    _resetUnusableInputWarningsForTests();
+    const emitted = emissionsDuring(() => {
+      warnUnusableInput({
+        reason: UNUSABLE_REASON.CONFIG_UNREADABLE,
+        source: '/u/config-unreadable-a/config.json',
+      });
+      warnUnusableInput({
+        reason: UNUSABLE_REASON.CONFIG_UNREADABLE,
+        source: '/u/config-unreadable-b/config.json',
+      });
+    });
+    assert.strictEqual(emitted, 2, 'keying too coarsely would hide a real second fault');
+  });
+
+  test('the reason value is the frozen string "config_unreadable"', () => {
+    assert.strictEqual(UNUSABLE_REASON.CONFIG_UNREADABLE, 'config_unreadable');
+  });
+});
+
 // ─── The discriminator: truncated vs. everything that merely looks like it ───
 
 describe('extractFrontmatter — flags a genuinely truncated frontmatter', () => {