refactor(main): add runtime domain registry and fan-in guardrails

Introduce runtime domain barrel exports and registry composition support, and document/check main runtime fan-in workflow with related backlog planning artifacts.

docs/subagents/agents/codex-task95-anki-20260221T031836Z-6f3e.md

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+# Agent: `codex-task95-anki-20260221T031836Z-6f3e`
+
+- alias: `codex-task95-anki`
+- mission: `Implement TASK-95 anki-integration extraction for field-grouping merge collaborator`
+- status: `done`
+- branch: `main`
+- started_at: `2026-02-21T03:18:36Z`
+- heartbeat_minutes: `5`
+
+## Current Work (newest first)
+
+- [2026-02-21T03:18:36Z] intent: extract field-grouping/merge collaborator from `src/anki-integration.ts`; add seam tests; run build + node tests; no commit.
+- [2026-02-21T03:26:55Z] completed: extracted collaborator to `src/anki-integration/field-grouping-merge.ts`; rewired `AnkiIntegration`; added seam tests; verified `bun run build && node --test dist/anki-integration.test.js`.
+
+## Files Touched
+
+- `docs/subagents/agents/codex-task95-anki-20260221T031836Z-6f3e.md`
+- `docs/subagents/INDEX.md`
+- `docs/subagents/collaboration.md`
+- `src/anki-integration.ts`
+- `src/anki-integration/field-grouping-merge.ts`
+- `src/anki-integration.test.ts`
+
+## Assumptions
+
+- TASK-95 scope in this session limited to Anki portion requested by user.
+- Keep `AnkiIntegration` external behavior unchanged; internal extraction only.
+
+## Open Questions / Blockers
+
+- None.
+
+## Next Step
+
+- Handoff ready: no blockers; no commit performed.