chore(claude): add /nxt slash command to report next tracker item

[officialCodeWork]

Summary

• Adds .claude/commands/nxt.md — a project-scoped slash command that reads TRACKER.md and reports the next actionable step.
• Output includes step number + title, phase, suggested branch name, dependency status, key deliverables, and any linked ADRs.
• The command is read-only: it does not create branches, files, or PRs — it just reports so you can decide whether to start.

Why

Every session begins with "what's next?" against TRACKER.md. Typing /nxt shortcuts that lookup and keeps the answer consistent (same fields every time) without burning model time re-deriving the format.

Notes for reviewers

• Lives under .claude/commands/ so it's checked in and available to teammates running Claude Code in this repo.
• No code changes, no CI impact — only a new markdown command definition.

Documentation

No docs/ update — this is tooling for the Claude Code workflow, not a product/API change. The command file itself is self-documenting via its frontmatter description and body.

Test plan

• Run /nxt in a fresh Claude Code session inside this repo and confirm it reports Phase 1 Step 1.1a as the next item.
• Confirm /nxt does not modify any files or create branches.

🤖 Generated with Claude Code