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.

src/unusable-input.cts

@@ -64,6 +64,13 @@ const UNUSABLE_REASON = Object.freeze({
    * planning-snapshot's current-phase field)
    */
   STATE_UNREADABLE: 'state_unreadable',
+  /**
+   * A config.json exists but could not be read/parsed (EACCES/EIO/malformed JSON/…).
+   * Distinct from a project that has not run any config-writing command yet: absence
+   * returns the same non-answer, silently — only an exists-but-unreadable config.json
+   * is corruption. (#3309, eighth #1879 site — planning-snapshot's config field)
+   */
+  CONFIG_UNREADABLE: 'config_unreadable',
 } as const);
 
 type UnusableReason = (typeof UNUSABLE_REASON)[keyof typeof UNUSABLE_REASON];
@@ -78,6 +85,8 @@ const REASON_PROSE: Readonly<Record<UnusableReason, string>> = Object.freeze({
     'last_activity in STATE.md is present but unparseable as a date; stale_activity fell back to false (idle-stranded suppressed)',
   [UNUSABLE_REASON.STATE_UNREADABLE]:
     'STATE.md exists but could not be read; the current-phase label fell back to unavailable',
+  [UNUSABLE_REASON.CONFIG_UNREADABLE]:
+    'config.json exists but could not be read or parsed; the config field fell back to unavailable',
 });
 
 // ─── Dedup state ──────────────────────────────────────────────────────────────