chore: consolidate dev skills into onsager-ai/dev-skills (#302)

Remove vendored copies of cross-repo dev-process skills (plan-dag,
agent-browser, github-integration, parallel-worktrees, issue-spec) —
these now install globally via 'npx skills add -g
onsager-ai/dev-skills --skill * -a claude-code'. The local
dev-process skills (lean-spec-dev-process, lean-spec-pre-push,
lean-spec-pr-lifecycle, leanspec-development, watch-ci) stay local.

- Drop .agents/skills/{plan-dag,agent-browser,github-integration,
  parallel-worktrees,issue-spec}
- Drop corresponding .claude/skills/* symlinks
- skills-lock.json becomes empty (all three pinned skills migrated)
- Rewrite local skill cross-links to removed skills as absolute URLs
  pointing at onsager-ai/dev-skills

Author: tikazyq