docs/skills.md missing 5 skills: checkpoint, health, pair-agent, devex-review, plan-devex-review

## Summary

`docs/skills.md` documents 31 skills but the repo ships 36 skill directories (excluding `connect-chrome` which is an alias for `open-gstack-browser`). **5 skills are undocumented.**

## Missing skills

| Skill | Added in | Description |
|-------|----------|-------------|
| `/checkpoint` | v0.15 | Save and resume working state snapshots. Captures git state, decisions made, remaining work. Supports cross-branch listing for Conductor workspace handoff. |
| `/health` | v0.15 | Code quality scorekeeper. Wraps project tools (tsc, biome, knip, shellcheck, tests), computes composite 0-10 score, tracks trends. |
| `/pair-agent` | v0.14+ | Pair a remote AI agent with your browser. Generates a setup key so OpenClaw, Hermes, Codex, Cursor, or any HTTP-capable agent can connect. |
| `/devex-review` | v0.14 | Live DX audit using the browse tool. Tests docs, getting started flows, error messages, CLI help. Boomerang comparison against `/plan-devex-review` scores. |
| `/plan-devex-review` | v0.14 | Plan-stage DX review. Rates 8 dimensions (getting started, API design, error messages, docs, upgrade path, dev environment, community, measurement) 0-10 with trend tracking. |

## How I found this

Diffed the skill names listed in `docs/skills.md` against the actual skill directories installed by gstack v0.15.15.1. All 5 missing skills have complete `SKILL.md` files with frontmatter and are fully functional — they're just not in the docs page.

## Notes

- The CHANGELOG already describes all 5 skills in their release entries — the descriptions and context exist, they just need to be added to `skills.md` in the same format as the existing entries (table row + deep-dive section).
- `/connect-chrome` shares its `SKILL.md` with `/open-gstack-browser` (name field = `open-gstack-browser`), so it's an alias, not a separate undocumented skill.

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