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>

capabilities/codebuddy/capability.json

@@ -35,6 +35,14 @@
           "nesting": "flat",
           "recursive": false,
           "converter": "convertClaudeCommandToCodebuddySkill"
+        },
+        {
+          "kind": "agents",
+          "destSubpath": "agents",
+          "prefix": "gsd-",
+          "nesting": "flat",
+          "recursive": false,
+          "converter": "convertClaudeAgentToCodebuddyAgent"
         }
       ],
       "local": [
@@ -53,6 +61,14 @@
           "nesting": "flat",
           "recursive": false,
           "converter": "convertClaudeCommandToCodebuddySkill"
+        },
+        {
+          "kind": "agents",
+          "destSubpath": "agents",
+          "prefix": "gsd-",
+          "nesting": "flat",
+          "recursive": false,
+          "converter": "convertClaudeAgentToCodebuddyAgent"
         }
       ]
     },