feat(#1035): phase 5a — author 16 role:runtime capability descriptors (registry-only) (#1039)

Author capabilities/<runtime>/capability.json for all 16 runtimes (role:runtime),
populating the registry's runtimes index ({} -> 16). Each carries the 6 ADR-1016
axes (configHome structured, configFormat, artifactLayout structured, commandStyle,
hooksSurface + hookEvents, sandboxTier, supportTier), extracted from the live
modules (runtime-homes/runtime-slash/runtime-artifact-layout/runtime-config-adapter
+ CODEX_AGENT_SANDBOX). validateRuntimeBody tightened to enforce the closed
vocabularies + structured configHome/artifactLayout (rejects old string configHome;
env required; skillsHome recursively validated).

Registry-only: bin/install.js + src/*.cts untouched, zero behavior change. The 5b-5f
drive steps consume these one axis at a time (ADR-1016 §8).

Codex accuracy review caught + fixed: opencode/kilo register ZERO lifecycle hooks
(install.js skips the whole block) -> hooksSurface:none (configFormat stays
settings-json); kimi probe selects on <root>/skills existence -> probeExists:"skills".
ArtifactKind required-field strictness + ConverterName closing deferred to 5d/5e.
ADR-1016 (Proposed) amended to match (Decisions 1 + 5).

Closes #1035

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

capabilities/opencode/capability.json

@@ -0,0 +1,41 @@
+{
+  "id": "opencode",
+  "role": "runtime",
+  "title": "OpenCode",
+  "description": "OpenCode — XDG-based config dir; flat command/ + skills artifact layout; settings-json config format; no lifecycle hook registration; tier-2 support.",
+  "tier": "core",
+  "requires": [],
+  "runtime": {
+    "configHome": {
+      "kind": "xdg",
+      "name": "opencode",
+      "env": ["OPENCODE_CONFIG_DIR", "OPENCODE_CONFIG", "XDG_CONFIG_HOME"]
+    },
+    "configFormat": "settings-json",
+    "artifactLayout": {
+      "global": [
+        {
+          "kind": "commands",
+          "destSubpath": "command",
+          "prefix": "gsd-",
+          "nesting": "flat",
+          "recursive": false,
+          "converter": null
+        },
+        {
+          "kind": "skills",
+          "destSubpath": "skills",
+          "prefix": "gsd-",
+          "nesting": "flat",
+          "recursive": true,
+          "converter": "convertClaudeCommandToOpencodeSkill"
+        }
+      ],
+      "local": []
+    },
+    "commandStyle": "slash-hyphen",
+    "hooksSurface": "none",
+    "sandboxTier": "none",
+    "supportTier": 2
+  }
+}