Keep .cursor/skills and .claude/skills in sync across create/review/preflight changes

## Context

`.claude/skills/` and `.cursor/skills/` currently hold duplicated (not symlinked) copies of the same skill files, e.g. `preflight-learning-path/SKILL.md`. The epic's goal is for these workflows to "work equally well invoked from Claude Code and Cursor," but nothing enforces that the two trees stay identical as #580, #471, #582, and the new preflight issues land.

## Scope

- Decide on a sync mechanism: symlink `.claude/skills/*` to `.cursor/skills/*` (or vice versa), a build/copy script, or a CI check that fails on drift
- Apply it, and verify current content is identical before switching
- Document the convention so future skill changes know which copy is canonical

## Success criteria

- [ ] `.claude/skills/` and `.cursor/skills/` cannot silently drift apart (symlinked, generated, or CI-checked)
- [ ] Convention documented for future skill changes

## Related

Part of epic #577.