chore: add git worktree docs and roborev configuration

[ian-flores]

Summary

• Add Git Worktrees section to CLAUDE.md mandating worktrees over plain branches for concurrent Claude sessions
• Add .envrc with source_up so direnv resolves PTD correctly in both main checkout and worktrees
• Add .roborev.toml configuring automatic code reviews with claude-code agent

Test plan

• Verify direnv allow works in main checkout and in a worktree
• Verify just build-cmd produces a working binary in a worktree
• Verify roborev triggers on commit (roborev status)

[Lytol]

All the roborev and CLAUDE.md stuff seems great! The .envrc will stomp on my own project-specific stuff, which feels heavy-handed. What do you think?

[Lytol]

Putting an envrc in the repo kind of takes up a typically developer-controlled local file. What about putting it in a .envrc.recommended or similar and then documenting via the README?