feat(#1708): typed documentation-sourced #853 dispatch-flatten (ADR-1239 Phase B) (#1719)

* feat(#1708): typed documentation-sourced #853 dispatch-flatten

Graduate the #853 orchestrator-backgrounding decision from a scattered RUNTIME==='codex' prose check to a typed, documentation-sourced engine decision. Adds a backgroundDispatch dispatch sub-axis (sourced per host: codex+cursor documented true, 9 documented false, 5 undocumented), shouldFlattenDispatch(dispatch) (inline UNLESS background && backgroundDispatch, fail-closed), and a gsd_run query dispatch-should-flatten the plan/execute workflows call. Cursor is newly background-eligible per its docs (inline->background) — a documentation-justified behavior change. No RUNTIME-name residue for this decision.

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

* docs(#1708): backgroundDispatch citations in matrix + CONTEXT note

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

* fix(#1708): address review findings on typed dispatch-flatten

Code/adversarial review: convert the manager.md/autonomous.md Compound Action preamble from hardcoded 'On Codex' to FLATTEN-based branching (the handlers already use the query; the preamble contradicted them and was wrong for cursor); make shouldFlattenDispatch null-safe + type-honest (accepts raw 'undocumented' registry values); make backgroundDispatch a required descriptor field (matching its siblings, all 16 carry it); strengthen the config.runtime behavioral test; update the bug-853 prose-pin test + comment. Security review clean; Codex confirmed no fail-open.

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

* test(#1708): backfill backgroundDispatch in role:runtime test fixtures

Making backgroundDispatch a required descriptor field broke role:runtime fixtures in capability-manifest-version/capability-registry/host-integration-descriptors tests that build a dispatch object without it (caught by full gsd-test, not scoped npm test). Backfill backgroundDispatch:false into the well-formed fixtures; the deliberately-malformed 'required-field' test fixture is left malformed by design.

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

* test(#1708): update fix-1521 dispatch-gating assertion to the FLATTEN gate

fix-1521 pinned the codex-specific run_in_background prose that #1708 graduated to the typed dispatch-should-flatten/FLATTEN gate. Update its assertions to verify FLATTEN=false gating (not a runtime name) + that the old RUNTIME===codex gate is gone. Caught by full gsd-test.

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

* docs(#1708): add changeset for typed dispatch-flatten

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

* chore(#1708): remove stray temp PR-body file

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

* test(#1708): add issue ref to bug-853 allow-test-rule annotations

ADR-456 requires every allow-test-rule exemption to carry a see #NNN reference; the source-text-is-the-product annotations added when migrating the prose assertions lacked it (lint-tests CI gate). Add (see #1708).

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

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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

capabilities/opencode/capability.json

@@ -69,7 +69,7 @@
     "hostIntegration": {
       "embeddingMode": "imperative",
       "commandSurface": "slash-file",
-      "dispatch": { "namedDispatch": true, "nested": "undocumented", "maxDepth": "undocumented", "background": false, "subagentToolkit": "full" },
+      "dispatch": { "namedDispatch": true, "nested": "undocumented", "maxDepth": "undocumented", "background": false, "subagentToolkit": "full", "backgroundDispatch": "undocumented" },
       "modelMode": "active",
       "hookBus": "host",
       "stateIO": "filesystem",