refactor(#1756): derive getDirName from a documented runtime.localConfigDir descriptor axis (#1757)

ADR-1239 Phase B (parent #1679). getDirName was a hand-maintained 15-branch
if-chain mapping each runtime to its local content-rewrite dot-dir. Relocate
those values into a documented runtime.localConfigDir descriptor field; derive
getDirName from registry.runtimes[id].runtime.localConfigDir (fallback .claude).

- 16 capability.json gain runtime.localConfigDir (byte-identical values)
- capability-validator.cjs requires it (non-empty dot-dir); registry regenerated
- docs/reference/capability-manifest.md documents the field + the three
  divergent values (copilot=.github, antigravity=.agents, kimi=.kimi-code)
- drift-guard test: golden value map + key-set equality both ways

Byte-identical install output for all 16 runtimes (golden-parity harness #1730).

Closes #1756

Co-authored-by: review-bot <review-bot@gsd>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/host-integration-validator-parity.test.cjs

@@ -51,6 +51,7 @@ function makeMinimalRuntimeCap(overrides = {}) {
       sandboxTier: 'none',
       supportTier: 1,
       installSurface: 'profile-marker-only',
+      localConfigDir: '.test-runtime',
       writesSharedSettings: false,
       permissionWriter: null,
       extendedHookEvents: [],
@@ -331,6 +332,7 @@ describe('Fix 3: reserved-key guard on hostIntegration and hostIntegration.dispa
       sandboxTier: 'none',
       supportTier: 1,
       installSurface: 'settings-json',
+      localConfigDir: '.test-runtime',
       writesSharedSettings: true,
       permissionWriter: null,
       extendedHookEvents: ['SubagentStop', 'Stop', 'PreCompact', 'FileChanged'],