feat(#1173): wire the 8 agent converters into the descriptor-driven install path

The descriptor-driven install path (resolveRuntimeArtifactLayout) installed no
agents for copilot/antigravity/cursor/windsurf/augment/trae/codebuddy/cline —
their per-runtime agent conversion ran only via the legacy bin/install.js loop.
This wires each runtime's agent converter into the descriptor so the new path
applies per-runtime conversion (follow-up to #1099; ADR-1235 cutover).

- capabilities/<rt>/capability.json: declare an `agents` kind with the runtime's
  converter (global+local; cline global-only). Regenerated capability-registry.cjs.
- convertedAgentsKind threads install scope -> isGlobal so the scope-aware
  copilot/antigravity converters choose global vs workspace-relative paths; the
  six single-arg converters ignore the extra arg. stageAgentsForRuntimeWithConverter
  passes isGlobal to the converter.
- Tests: feat-1173 gains a real-registry block asserting each runtime's descriptor
  applies the correct converter (== conv(src, isGlobal), != raw copy) with scope
  threading (fails-first on pristine next). The ADR-857 equivalence golden +
  per-runtime kind-count assertions now include the agents kind, each annotated as
  an intentional #1173 change.

Scope: this wires the per-runtime CONVERTER. The remaining byte-parity behaviors of
the legacy loop (copilot `.agent.md` rename, cross-cutting path/attribution rewrites,
config-reading) stay with the legacy loop -- which runs after installRuntimeArtifacts
and is authoritative for the real install -- and are tracked by ADR-1235's later
cutover steps. No user-facing change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/enh-789-codebuddy-commands.test.cjs

@@ -53,11 +53,12 @@ const RESOLVED_CORE = resolveProfile({ modes: ['core'], manifest: MANIFEST });
 // ─── Layout contract ─────────────────────────────────────────────────────────
 
 describe('enh-789 — codebuddy layout has commands + skills kinds', () => {
-  test('resolveRuntimeArtifactLayout codebuddy returns 2 kinds', () => {
+  test('resolveRuntimeArtifactLayout codebuddy returns 3 kinds', () => {
+    // #1173: codebuddy gained an agents kind (descriptor-driven per-runtime agent conversion).
     const layout = resolveRuntimeArtifactLayout('codebuddy', '/tmp/fake-codebuddy-dir');
-    assert.strictEqual(layout.kinds.length, 2, 'codebuddy must have exactly 2 artifact kinds');
+    assert.strictEqual(layout.kinds.length, 3, 'codebuddy must have exactly 3 artifact kinds');
     const kindNames = layout.kinds.map(k => k.kind).sort();
-    assert.deepStrictEqual(kindNames, ['commands', 'skills']);
+    assert.deepStrictEqual(kindNames, ['agents', 'commands', 'skills']);
   });
 
   test('codebuddy commands kind targets commands/ with gsd- prefix', () => {