fix: add workspace skill root to skill-sync.ts (#410)

[Brixyy]

Summary

• Adds `workspace` root to `getSkillRoots()` (`src/lib/skill-sync.ts`): mirrors the root added to `route.ts` in feat: workspace skill root and root-based filtering in Skills Hub #408; the scheduler now discovers and syncs skills from `~/.openclaw/workspace/skills` (or `MC_SKILLS_WORKSPACE_DIR` / `OPENCLAW_WORKSPACE_DIR` overrides)
• Adds `'workspace'` to `localSources` (`syncSkillsFromDisk()`): the DB query that checks for existing rows before upsert now includes the workspace source, preventing spurious re-inserts on every sync cycle
• Updates doc comment from "Scans 4 skill roots" to "Scans 5 skill roots"

Closes #410

Test plan

• Verified locally: placed skills with `SKILL.md` in `~/.openclaw/workspace/skills/`, triggered scheduler via `POST /api/scheduler` with `task_id=skill_sync` — skills appeared in DB and Skills Hub showed correct count (10 workspace skills)
• Confirmed `~/.openclaw/workspace/skills` card updates from 0 → 10 after sync
• `pnpm typecheck` — no errors
• `pnpm test` — all pass
• `pnpm lint` — no new warnings

[0xNyk]

Clean, surgical fix for the regression from #408. The workspace source was added to route.ts but missed in skill-sync.ts, so the scheduler sync never populated workspace skills.

LGTM — merging first so #413 can build on it.