feat(architecture): [EoS/opencode] Migrate OpenCode onto the Embeddable Orchestration System (ADR-1239, #2087)

Route OpenCode (and its Kilo sibling) through the public Host-Integration Interface and
land two Context7-verified capability upgrades. Byte-identical install output for all 16
runtimes (golden parity asserted).

Through the interface (AC2):
- OpenCode/Kilo's bespoke commands+skills+plugin install (the inline
  `else if (isOpencode || isKilo)` block) moves into the engine
  (installOpencodeFamilyCommands/Artifacts in src/install-engine.cts), dispatched by
  installRuntimeArtifacts when the descriptor declares hostBehaviors.combinedFamilyInstall.
  opencode/kilo now flow CLI -> _runtimeAdapter -> installRuntimeArtifacts like the skills
  runtimes. _isSkillsRuntime no longer excludes them; the bespoke block + dead
  copyFlattenedCommands are removed.
- Every hardcoded `runtime === 'opencode'`/`isOpencode` branch is folded into
  descriptor-driven runtime.hostBehaviors. ZERO `runtime === 'opencode'`/`'kilo'`
  string-equality remain in bin/install.js / install-engine.cts / runtime-artifact-conversion.cts.

Upgrades (AC4):
- Background dispatch: OpenCode shipped experimental background subagents in v1.15 and
  made them default-on in v1.17 -> dispatch.background/backgroundDispatch flip to true;
  shouldFlattenDispatch(opencode) now returns false (behavioral change; type: Changed).
- Expanded event surface: the OpenCode plugin subscribes permission.asked/replied +
  session.error.

Tests: opencode-imperative-reference (adapter/profile, shouldFlattenDispatch pin,
fail-closed negotiate, hostBehaviors, AC2 source-guard) + extended plugin surface test.
Docs: capability matrix v1.15/v1.17 citations. Changeset (Changed). gitignore .memdb//.memtrace/.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

capabilities/opencode/capability.json

@@ -75,15 +75,31 @@
         "namedDispatch": true,
         "nested": "undocumented",
         "maxDepth": "undocumented",
-        "background": false,
+        "background": true,
         "subagentToolkit": "full",
-        "backgroundDispatch": "undocumented"
+        "backgroundDispatch": true
       },
       "modelMode": "active",
       "hookBus": "host",
       "stateIO": "filesystem",
       "transport": "mcp",
       "runtime": "bun"
+    },
+    "hostBehaviors": {
+      "reapplyCommand": "/gsd-update --reapply",
+      "attributionConfigResolver": "opencode",
+      "flatCommandDir": "command",
+      "combinedFamilyInstall": true,
+      "frontmatterDialect": "opencode",
+      "nativePlugin": {
+        "dir": "plugins",
+        "file": "gsd-core.js",
+        "source": ".opencode/plugins/gsd-core.js"
+      },
+      "skipHomePrefixSubstitution": true,
+      "skipSettingsUi": true,
+      "skipUpdateBannerCommand": true,
+      "skipCodexSkillsManifest": true
     }
   }
 }