refactor(#1173): defer agents-kind descriptor declarations (option a)

Adopt maintainer-recommended option (a): keep the convertedAgentsKind /
stageAgentsForRuntimeWithConverter scope-threading plumbing, but DEFER the
8 runtimes' capability.json `agents`-kind declarations to a follow-up that
first ships the ADR-1235 §0 byte-for-byte parity harness.

The declarations were a live regression: the second `layout.kinds` consumer,
applySurface / `/gsd:surface` / `--materialize` (src/surface.cts), does not
mirror the legacy agent pipeline (copilot `.agent.md` rename, path-prefix
rewrite + attribution, stale cleanup), so a `/gsd:surface` toggle deleted
installed copilot `gsd-*.agent.md` and path-unrewrote the other 7 runtimes.
trek-e + davesienkowski both flagged this.

- Revert the agents-kind entries from the 8 capability.json files and
  regenerate capability-registry.cjs (now matches next; 0 converted agents
  kinds declared).
- Revert the declaration-driven kind-count test bumps
  (runtime-artifact-layout, descriptor-drive, bug-782, enh-789, enh-790).
- Keep the synthetic-descriptor seam tests for convertedAgentsKind dispatch;
  add a synthetic scope-threading test so the kept isGlobal plumbing stays
  covered without depending on real declarations.
- Update the convertedAgentsKind doc comment to state declarations are
  deferred pending the ADR-1235 §0 parity harness.
- Move ADR-1235 to Accepted.
- Reword the changeset to plumbing-only (docs-exempt now honest: no runtime
  declares the kind, legacy loop authoritative, installed output unchanged).

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

docs/adr/1235-descriptor-driven-agent-conversion-migration.md

@@ -1,6 +1,6 @@
 # ADR-1235: Migrate agent conversion to the descriptor-driven install path
 
-- **Status:** Proposed
+- **Status:** Accepted
 - **Date:** 2026-06-14
 - **Issue:** #1235
 - **Builds on:** [ADR-3660](3660-runtime-artifact-layout-module.md) (runtime artifact layout), [ADR-457](457-generated-cjs-single-source.md) (the `src/*.cts` build-at-publish tree the converters live in), [ADR-1016](1016-runtime-capability-descriptor.md) (runtime capability descriptor)