Playbook 06 — one feature, one branch, one MR per repo

Records the branching-and-merging workflow for code changes across the mesh
repos, written against a failure it names: branches and MRs opened per unit of
thought, treated as done when opened not merged, and named differently per repo,
so they pile up unmerged — one session left sixteen to consolidate by hand. The
rule is one feat/<slug> shared across every repo a feature touches, isolated in
.work/<slug>/<repo> worktrees off main, pushed and opened as one MR per repo only
when the whole feature is done, then merged promptly. Adds the ground-rule
pointer in AGENTS.md and the row in the process overview.

Claude-Session: https://claude.ai/code/session_01LrgweAeERJYBg88c5cKDzF

00-META/process/00-overview.md

@@ -49,6 +49,7 @@ the expensive half.
 | [03](03-issues.md) | Issues | Something is wrong — often with the owner unknown |
 | [04](04-build-handoff.md) | Build handoff | A design is ready to be built |
 | [05](05-constitution-sync.md) | Constitution sync | `how-we-build.md` changed a rule the mesh enforces |
+| [06](06-feature-branches.md) | Feature branches across repos | Code changes, in one repo or several, need branching and merging |
 
 ## Status lives in frontmatter