Add esper:done skill — finalize a plan with a guaranteed commit

Context

Currently the esper workflow has two commit-related commands:

• /esper:commit — save partial progress anytime; only commits if there are changes
• /esper:ship — verify + commit + push + open PR + archive

There is no command that finalizes a plan (archives it to done/) without opening a PR. This gap matters most in type: "feature" plans, where per-plan PRs are skipped — the user needs a way to say "this plan is complete, commit my work and archive it" before moving on to the next plan. The phase PR is opened later by esper:ship when all phase plans are done.

The key difference from /esper:commit:

• /esper:commit: partial save, can be called anytime, no-ops if nothing changed
• /esper:done: finalization step — always produces a commit (even if the only change is the plan file itself), then archives the plan to done/

Files changed

• skills/esper-done/SKILL.md (created — the new skill)
• skills/esper-ship/SKILL.md (modified — Step 1: check done/ fallback; Step 6: handle already-archived plans)

Verification

• Run: manual
• Expected:
  • Feature plan workflow: implement plan → /esper:done → plan is in done/ with status: done, shipped_at set, no pr field; working tree committed → repeat for next plan → /esper:ship finds done plans, opens phase PR, adds pr: to each plan's frontmatter
  • Fix plan workflow: implement → /esper:done → /esper:ship finds the done plan, pushes, opens PR, updates plan frontmatter with pr: URL
  • /esper:commit behavior unchanged

Progress

• Created skills/esper-done/SKILL.md: 5-step workflow — check setup → verify → commit (always, skip if clean) → archive to done/ (no pr field) → summary with type-aware next-step message
• Updated skills/esper-ship/SKILL.md Step 1: checks done/ for unshipped plan (no pr field) if active/ is empty; updated Step 6: handles already-archived plans in done/ by only updating pr field instead of moving
• Adapted plan to use type field instead of deprecated pr_mode references
• Modified: skills/esper-done/SKILL.md (created), skills/esper-ship/SKILL.md
• Verification: manual — new skill visible via node bin/cli.js