docs(changeset): backfill pr 2108 for #2087 changeset

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

.changeset/2087-eos-opencode-imperative-adapter.md

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 type: Changed
-pr: 2087
+pr: 2108
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 **OpenCode is now driven through the public Host-Integration Interface, with two capability upgrades (ADR-1239 / EoS).** OpenCode and its Kilo sibling previously installed via a bespoke `runtime === 'opencode'`/`isOpencode` branch in `bin/install.js`; its commands+skills+plugin install now runs through the imperative adapter → the engine's combined-family install path (`installRuntimeArtifacts`), and every hardcoded `runtime === 'opencode'` branch is folded into descriptor-driven `runtime.hostBehaviors`. Install/uninstall output is **byte-identical** (golden parity asserted for all 16 runtimes). Two Context7-verified upgrades land: (1) **background dispatch** — OpenCode shipped experimental background subagents in v1.15 and made them default-on in v1.17, so `dispatch.background`/`backgroundDispatch` flip to `true`; GSD no longer force-flattens OpenCode-hosted wave dispatch (`shouldFlattenDispatch` now returns `false`), letting agents run concurrently where the host supports it. (2) **expanded event surface** — the OpenCode plugin now subscribes to `permission.asked`, `permission.replied`, and `session.error` (added to `EXTENSION_EVENT_SURFACES.opencode`), wiring the declared surface for future permission/error-aware bindings. (#2087)