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/opencode-task95-immersion-tracker-20260221T031846Z-p4k9.md

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+# Agent Session: opencode-task95-immersion-tracker-20260221T031846Z-p4k9
+
+- alias: `opencode-task95-immersion-tracker`
+- mission: `Implement TASK-95 immersion-tracker extraction into focused collaborators and seam tests`
+- status: `handoff`
+- started_utc: `2026-02-21T03:18:46Z`
+- backlog_ticket: `TASK-95`
+
+## Intent
+
+- reduce `src/core/services/immersion-tracker-service.ts` LOC; preserve public behavior.
+- extract focused modules under `src/core/services/immersion-tracker/` (types/reducer/query/maintenance/queue helpers).
+- add/update seam tests in `src/core/services/immersion-tracker-service.test.ts`.
+
+## Planned Files
+
+- `src/core/services/immersion-tracker-service.ts`
+- `src/core/services/immersion-tracker/*`
+- `src/core/services/immersion-tracker-service.test.ts`
+
+## Assumptions
+
+- existing TASK-95 plan allows independent immersion-tracker slice.
+- no backlog file edits requested.
+
+## Phase Log
+
+- `2026-02-21T03:18:46Z` start; context loaded; beginning code/test extraction.
+- `2026-02-21T03:26:51Z` refactor complete; extracted `types/reducer/query/maintenance/queue` modules; added seam tests; ran `bun run build && node --test dist/core/services/immersion-tracker-service.test.js`.
+
+## Handoff
+
+- touched: `src/core/services/immersion-tracker-service.ts`, `src/core/services/immersion-tracker-service.test.ts`, `src/core/services/immersion-tracker/*`.
+- behavior guardrails: kept `ImmersionTrackerService` public API and DB schema/event constants stable.
+- note: sqlite-backed tests skip in this environment; seam unit tests run and pass.