feat: [ENG-2536] add ByteRover onboarding tour skill

Add a guided 90-second onboarding skill that runs when a user asks
"Show me how ByteRover works" (or any semantic equivalent). The tour
captures the user's persona and the pain that brought them to
ByteRover, persists both locally, and demonstrates how ByteRover
ends the re-explain tax — all without an account, API key, or
provider config.

src/server/templates/skill/onboarding.md (new):
- Msg 1: trust opener (no command names) → one-sentence concept
  primer → quick interview (identity + pain in one open question) →
  human-reaction beat before curate → visible artifact with pain
  bullet + sticky label ("re-explain tax") + behavioral commitment
  → 2-beat concept map with visual separation before pause
- Msg 2: named action ("brv query"), one-line retrieval summary,
  conditional bonus-context branch when existing project knowledge
  is found, identity-and-pain reflection, future-self demonstration
  of the pain ending, loop name + cross-session promise
- Msg 3: where memory lives (one sentence, no commands) →
  activation step that detects CLAUDE.md/AGENTS.md/etc. and offers
  to curate as starter project context (or prompts for one rule)
  → done signal with two equal-weight paths (curate more vs.
  resume work) so users exit without homework
- Spec guards: don't manufacture pains, don't overpromise on
  out-of-scope pains, don't turn the artifact into a confirmation,
  don't tailor with hollow phrases, don't auto-fire next message

src/server/templates/skill/SKILL.md:
- Add First-Turn Routing block before the Iron Law that hands off
  to onboarding.md on tour/intro/walkthrough requests
- Update frontmatter description so the skill router activates the
  skill on tour-related phrasings

src/server/infra/connectors/skill/skill-connector-config.ts:
- Add onboarding.md to SKILL_FILE_NAMES so the connector copies it
  during install