fix: [ENG-2697] sub-skill gaps — missing brv commands + examples

Audited each sub-skill (excluding curate / query) against the actual
`brv <topic> --help` surface and patched the gaps:

- vc.md: added init, clone, config, checkout, merge, fetch, remote;
  added real examples for reset (was only mentioned as "avoid").
  Regrouped Quick Reference by workflow phase (Setup / Inspect /
  Stage+commit / Branch+checkout / Sync / Merge / Undo). Expanded
  Safety + Common Mistakes to cover missing-upstream, missing-config,
  and switching with dirty tree.
- swarm.md: added `brv swarm onboard` (interactive setup wizard) +
  a Setup section explaining when to use it.
- review.md: added the toggle subcommands (`brv review`,
  `--enable`, `--disable`) that the description verb already implied
  but the body missed entirely. Updated frontmatter to include
  enable/disable so the matcher fires on those prompts.
- history.md: surfaced the filter flags that were buried under
  `--help` — `--limit`, `--since`, `--before`, `--status`, `--tier`,
  `--last`, `--format narrative`. Regrouped Quick Reference into
  Curate / Query / Aggregate sections.
- troubleshooting.md: added recovery commands (`brv restart` for
  crash recovery, `brv logout` + `brv login` reset flow, `brv update`
  for stale-CLI mismatch) + `brv connectors` to Quick Diagnosis.