Add esper:review skill for code quality review

Context

esper currently has no skill for reviewing code before shipping. The /esper:ship skill pushes and opens a PR, but there is no structured code review step. An esper:review skill would let users get code quality feedback on their branch diff before shipping, catching issues early.

The skill should work at the branch/PR level — reviewing the diff between the current branch and main, focused on code quality (bugs, style, security, complexity).

Approach

1. Create skills/esper-review/SKILL.md with the following workflow:

   • Step 1: Check setup — run esperkit config check, determine current branch and base branch
   • Step 2: Gather diff — run git diff main...HEAD (or the appropriate base) to get the full branch diff. If a PR number is provided as argument, use gh pr diff <number> instead
   • Step 3: Read context — read .esper/CONSTITUTION.md for project principles. If there's an active plan, read it for intent context (what the code is supposed to do)
   • Step 4: Review — analyze the diff for:
     • Bugs and logic errors
     • Security issues (OWASP top 10)
     • Code style and consistency with existing patterns
     • Missing error handling or edge cases
     • Unnecessary complexity or dead code
   • Step 5: Report — print a structured review with sections for each category. Use severity levels (critical, warning, note). Include file paths and line references
   • Step 6: Offer fixes — if actionable issues were found, use AskUserQuestion to ask if the user wants to create /esper:fix plans for them. If yes, batch the selected issues into fix plans (one per issue or grouped by file/concern, at the user's preference)

2. No changes to bin/cli.js — the installer auto-discovers new skill directories.

Files to change

• skills/esper-review/SKILL.md (create — the skill definition)

Verification

• Run: node --test test/
• Expected: existing tests still pass
• Edge cases: handle empty diffs (nothing to review), very large diffs (summarize rather than line-by-line), and PRs with no active plan context

Progress

• Created skills/esper-review/SKILL.md with 6-step workflow (check, gather diff, read context, review, report, offer fixes)
• Modified: skills/esper-review/SKILL.md
• Verification: 64/64 tests pass