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-discord-presence-task-20260222T194048Z-d7k2.md

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+# Agent: `codex-discord-presence-task-20260222T194048Z-d7k2`
+
+- alias: `codex-discord-presence-task`
+- mission: `Add backlog task for Discord Rich Presence integration with polished activity card`
+- status: `done`
+- branch: `main`
+- started_at: `2026-02-22T19:40:48Z`
+- heartbeat_minutes: `5`
+
+## Current Work (newest first)
+- [2026-02-22T19:41:00Z] handoff: added `TASK-109` backlog ticket for Discord Rich Presence integration + polished activity card scope.
+- [2026-02-22T19:40:56Z] progress: verified no existing Discord presence backlog ticket via search across backlog directories.
+- [2026-02-22T19:40:48Z] intent: create ticket only; no runtime code implementation.
+
+## Files Touched
+- `backlog/tasks/task-109 - Add-Discord-Rich-Presence-integration-with-polished-activity-card.md`
+- `docs/subagents/INDEX.md`
+- `docs/subagents/agents/codex-discord-presence-task-20260222T194048Z-d7k2.md`
+
+## Assumptions
+- User request means backlog task creation, not immediate implementation.
+- Existing markdown task format in `backlog/tasks` remains canonical.
+
+## Open Questions / Blockers
+- None.
+
+## Next Step
+- Wait for user follow-up (implement `TASK-109` when requested).