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>

docs/reference/host-integration-capability-matrix.md

@@ -134,9 +134,9 @@ Documentation gaps:
 | dispatch.namedDispatch | true | https://opencode.ai/docs/agents | "\"Subagents can be invoked: Automatically by primary agents for specialized tasks based on their descriptions. Manually b" |
 | dispatch.nested | undocumented | no authoritative doc — searched: https://opencode.ai/docs/agents | — |
 | dispatch.maxDepth | undocumented | no authoritative doc — searched: https://opencode.ai/docs/agents | — |
-| dispatch.background | false | https://github.com/sst/opencode/issues/5887 | "\"Currently, sub-agent delegation in `opencode` appears to be synchronous or modal... There is no native 'fire-and-forget'" |
+| dispatch.background | true | https://github.com/anomalyco/opencode/blob/dev/packages/opencode/src/tool/task.ts (v1.15.0, commit 22de34c4d) + src/effect/runtime-flags.ts (v1.17, commit 81f6e0668) | "New in v1.15.0: experimental background subagents — the Task tool gains a `background` parameter (`Schema.optional(Schema.Boolean)`) that launches subagents asynchronously with completion notifications. v1.17: `BACKGROUND_SUBAGENTS_ENABLED = true` (\"feat: enable background subagents by default\") — default-on, concurrent execution in all modes. (#2087, superseding the stale sst/opencode#5887 snapshot)" |
 | dispatch.subagentToolkit | full | https://opencode.ai/docs/agents | "The 'general' subagent \"Has full tool access (except todo), so it can make file changes when needed.\"" |
-| dispatch.backgroundDispatch | undocumented | no authoritative doc — https://github.com/anomalyco/opencode/issues/18100 and https://github.com/anomalyco/opencode/blob/dev/opencode/packages/opencode/src/tool/task.ts | Opencode supports background task dispatch via the Task tool's `background: true` parameter but whether a background-spawned agent can itself spawn further sub-agents is not documented. |
+| dispatch.backgroundDispatch | true | https://github.com/anomalyco/opencode/blob/dev/packages/opencode/src/effect/runtime-flags.ts (v1.17, commit 81f6e0668) + src/server/routes/instance/httpapi/handlers/experimental.ts | "v1.17 `BACKGROUND_SUBAGENTS_ENABLED = true` enables background subagent execution by default in all modes; the experimental capabilities endpoint exposes `{ backgroundSubagents: true }`. Background-spawned subagents run concurrently without blocking the main interaction flow. (#2087)" |
 
 Sources consulted:
 - https://opencode.ai/docs/plugins