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>

.opencode/plugins/gsd-core.js

@@ -663,6 +663,26 @@ const GsdCorePlugin = async ({ directory } = {}) => {
       if (event.type === "session.idle") {
         return;
       }
+
+      // permission.asked / permission.replied — OpenCode permission lifecycle
+      // (#2087, opencode.ai/docs/plugins). GSD gates tool INPUTS at
+      // tool.execute.before (read-guard, injection-scanner); the permission
+      // grant/deny decision itself carries no GSD workflow-phase contribution,
+      // so these are recognized sentinels — wired so a future permission-aware
+      // gate can attach without a plugin change (the engine owns phase
+      // sequencing; this host bus is session/tool/permission-scoped, never
+      // phase-scoped — ADR-1239 §OpenCode).
+      if (event.type === "permission.asked" || event.type === "permission.replied") {
+        return;
+      }
+
+      // session.error — OpenCode session-error lifecycle point (#2087). No GSD
+      // hook fires here today (loop state is already persisted to .planning/);
+      // recognized so the declared extension-event surface is fully wired and a
+      // future error-class hook can attach without a plugin change.
+      if (event.type === "session.error") {
+        return;
+      }
     },
   };
 };