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-task105-sliceb-20260222T195423Z-w8n3.md

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+# Agent Session: codex-task105-sliceb-20260222T195423Z-w8n3
+
+- alias: `codex-task105-sliceb`
+- mission: `Implement TASK-105 slice B runtime cast removal in targeted main/runtime modules without commit.`
+- status: `done`
+- started_utc: `2026-02-22T19:54:23Z`
+- last_update_utc: `2026-02-22T20:02:06Z`
+
+## Intent
+
+- Remove unsafe non-test casts in requested runtime modules using strict shared contracts.
+- Keep behavior unchanged; limit scope to type-safety and minimal fallout tests.
+- Run targeted tests for changed modules and report outputs.
+
+## Planned Files
+
+- `src/main/runtime/app-runtime-main-deps.ts`
+- `src/main/runtime/cli-command-context-main-deps.ts`
+- `src/main/runtime/subtitle-tokenization-main-deps.ts`
+- `src/main/runtime/overlay-runtime-options-main-deps.ts`
+- `src/main/runtime/dictionary-runtime-main-deps.ts`
+- `src/main/runtime/mpv-jellyfin-defaults.ts`
+- `src/main/runtime/jellyfin-playback-launch.ts`
+- `src/main/runtime/cli-command-context-main-deps.test.ts`
+- `src/main/runtime/subtitle-tokenization-main-deps.test.ts`
+- `src/main/runtime/mpv-jellyfin-defaults.test.ts`
+- `src/main/runtime/mpv-jellyfin-defaults-main-deps.test.ts`
+- `src/main/runtime/jellyfin-playback-launch.test.ts`
+- `src/main/runtime/jellyfin-playback-launch-main-deps.test.ts`
+- `src/main/runtime/**/*.test.ts`
+
+## Files Touched
+
+- `docs/subagents/INDEX.md`
+- `docs/subagents/agents/codex-task105-sliceb-20260222T195423Z-w8n3.md`
+- `docs/subagents/collaboration.md`
+- `src/main/runtime/app-runtime-main-deps.ts`
+- `src/main/runtime/cli-command-context-main-deps.ts`
+- `src/main/runtime/subtitle-tokenization-main-deps.ts`
+- `src/main/runtime/overlay-runtime-options-main-deps.ts`
+- `src/main/runtime/mpv-jellyfin-defaults.ts`
+- `src/main/runtime/jellyfin-playback-launch.ts`
+
+## Assumptions
+
+- Existing runtime contract types already cover needed dependency boundaries.
+- Requested slice B is limited to listed files plus minimal typing-test fallout.
+
+## Phase Log
+
+- `2026-02-22T19:54:23Z` Session started; loaded backlog workflow + task context + subagent docs; preparing cast hotspot edits.
+- `2026-02-22T19:59:42Z` Completed slice B edits: removed unsafe casts in targeted runtime modules, tightened contracts to shared runtime types, and ran targeted runtime tests (16 pass / 0 fail).
+- `2026-02-22T20:02:06Z` Updated focused tests for stricter contracts (typed stubs/status snapshots) and re-ran targeted suite (16 pass / 0 fail).
+
+## Next Step
+
+- Handoff ready.