feat(core): add Discord presence service and extract Jellyfin runtime composition

Introduce Discord presence runtime support and continue composition-root decomposition by moving Jellyfin wiring into dedicated composer modules. This keeps main runtime orchestration thinner while preserving behavior and test coverage across config, runtime, and docs updates.

docs/subagents/agents/codex-kiku-modal-overlay-20260222T220502Z-r4m1.md

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+# Agent: `codex-kiku-modal-overlay-20260222T220502Z-r4m1`
+
+- alias: `codex-kiku-modal-overlay`
+- mission: `Fix Kiku field-grouping modal overlay visibility restore when modal closes`
+- status: `handoff`
+- branch: `main`
+- started_at: `2026-02-22T22:05:02Z`
+- heartbeat_minutes: `5`
+
+## Current Work (newest first)
+- [2026-02-22T22:07:38Z] handoff: added failing regression test for hidden->modal->close restore path; patched `field-grouping-overlay` to sync visible overlay state for external senders; focused tests green.
+- [2026-02-22T22:05:02Z] intent: investigate Kiku modal + overlay visibility restore regression; add failing test first, then minimal fix.
+- [2026-02-22T22:06:10Z] progress: traced leak to `createFieldGroupingOverlayRuntime` path using `sendToActiveOverlayWindow` without syncing `visibleOverlayVisible` state.
+
+## Files Touched
+- `docs/subagents/INDEX.md`
+- `docs/subagents/collaboration.md`
+- `docs/subagents/agents/codex-kiku-modal-overlay-20260222T220502Z-r4m1.md`
+- `src/core/services/field-grouping-overlay.ts`
+- `src/core/services/field-grouping-overlay.test.ts`
+
+## Assumptions
+- Bug reproduces when visible overlay is initially hidden and Kiku modal auto-opens overlay.
+- Existing callback restore path should remain source of truth (hide on resolve/cancel).
+
+## Open Questions / Blockers
+- none
+
+## Next Step
+- User validation in real overlay flow (hidden visible overlay -> Kiku modal open/close -> hidden restored).