docs(dev-practices): add multi-session-worktrees-guide.md

[rafael5]

Summary

Companion to parallel-multi-repo-git-hygiene.md Rule 2. Where the strategic doc names the rule, this guide is the operational manual:

• Layout convention: sibling-of-main-checkout, <repo>-<branch-slug> naming
• Three setup variants (existing local branch, new branch, remote-only branch)
• Daily usage and per-worktree git fetch pattern
• Cleanup and git worktree prune
• Six gotchas: direnv, .venv/, pytest cache pinning, branch-already-checked-out errors, CI orthogonality, "don't run sessions in the main checkout"
• Bootstrap script for setting up worktrees across the existing org

Also wires Rule 2 of the strategic doc and the dev-practices/README.md index to point at this guide.

The session that authored this PR itself fell into the multi-session trap mid-flight (commit briefly landed on main because another session ran git checkout main in the same working tree between checkout -b and commit) — exactly the failure mode this guide is meant to prevent.

Test plan

• make check-docs-prose passes
• Links from parallel-multi-repo-git-hygiene.md §Rule 2 and dev-practices/README.md index resolve
• Frontmatter conforms to docs-discoverability spec

🤖 Generated with Claude Code

[rafael5]

Closing — worktrees are an escape hatch, not the default. Adopting the simpler 'one session per repo' rule (Rule 1 of parallel-multi-repo-git-hygiene.md) as the front-and-center practice instead. Worktrees stay documented as a one-line option inside the strategic doc for the rare case they're needed. The cognitive overhead of layout convention + setup commands + cleanup + gotchas isn't worth it when the simpler rule covers the same problem.