feat(gateway): add plugin host module

docs/scratchpads/41-plugin-host.md

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+# Scratchpad — P5-001 Plugin Host
+
+- Task: P5-001 / issue #41
+- Branch: feat/p5-plugin-host
+- Objective: add global NestJS plugin host module, wire Discord import, register active plugins from env, and attach to AppModule.
+- TDD: skipped as optional for module wiring/integration work; relying on targeted typecheck/lint and module-level situational verification.
+- Constraints: ESM .js imports, explicit @Inject(), follow existing gateway patterns, do not touch TASKS.md.
+
+## Progress Log
+
+- 2026-03-13: session started in worktree; loading gateway/plugin package context.
+- 2026-03-13: implemented initial plugin module, service, interface, and AppModule wiring; pending verification.
+- 2026-03-13: added `@mosaic/discord-plugin` as a gateway workspace dependency and regenerated `pnpm-lock.yaml`.
+- 2026-03-13: built gateway dependency chain so workspace packages exported `dist/*` for clean TypeScript resolution in this fresh worktree.
+- 2026-03-13: verification complete.
+
+## Verification
+
+- `pnpm --filter @mosaic/gateway... build` ✅
+- `pnpm --filter @mosaic/gateway typecheck` ✅
+- `pnpm --filter @mosaic/gateway lint` ✅
+- `pnpm format:check` ✅
+- `pnpm typecheck` ✅
+- `pnpm lint` ✅
+
+## Review
+
+- Automated review: `~/.config/mosaic/tools/codex/codex-code-review.sh --uncommitted`
+- Manual review: diff inspection of gateway plugin host changes
+- Result: no blocker findings