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
 

00-META/process/06-feature-branches.md

@@ -0,0 +1,57 @@
+# Playbook 06 — Feature branches across repos
+
+**Trigger.** Work that changes code — in one code repo or in several at once (`mesh-sdk`,
+`mesh-control`, `mesh-catalog`, `mesh-host`, `mesh-lab`, and `hq` when a decision rides along).
+
+**Who runs it.** Anyone who writes code, engineers and agents alike. Agents follow it exactly —
+it is the guard against the failure it was written for.
+
+## The failure it prevents
+
+A feature was worked as a branch-and-MR per *unit of thought* — one per decision, one per
+stacked increment — and each MR was treated as finished when it was *opened*, not when it was
+*merged*. Across repos the same feature took a different branch name in each. The MRs piled up
+unmerged: one session left **sixteen** stacked intermediate MRs that had to be consolidated and
+closed by hand. An MR is a review checkpoint, not a scratchpad.
+
+## The rule
+
+One feature is **one branch name**, **one worktree per repo**, **one MR per repo**, opened
+**once, at the end**.
+
+1. **Name the feature once.** `feat/<slug>`. The *same* branch name in every repo the feature
+   touches — never a different name per repo, never a fresh branch per increment within the
+   feature.
+2. **Isolate each repo.** One git worktree per touched repo under `.work/<slug>/<repo>`, branched
+   off `main`:
+   ```
+   git worktree add .work/<slug>/<repo> -b feat/<slug> origin/main
+   ```
+   Parallel features never collide, and no shared checkout is edited.
+3. **Commit as you go — locally.** Increments land on the one branch. Nothing is pushed and no
+   MR is opened mid-feature.
+4. **Finish, then publish.** When the whole feature is done — every repo, tests green — push
+   every branch and open **one MR per touched repo**, together.
+5. **Merge promptly, once approved.** Every merge into `main` is notified and approved
+   ([ADR 0042](../../02-DECISIONS/0042-approval-is-the-checkpoint.md)); once it is, merge —
+   do not leave it sitting. The branch is deleted on merge.
+6. **Leave nothing behind.** After the MRs merge, no `feat/<slug>` branch and no `.work/<slug>`
+   worktree survive.
+
+## What this is not
+
+- **Not a licence to batch unbounded work.** A feature is a *bounded* unit; if it sprawls for
+  days, end-of-feature bloat merely replaces per-increment bloat. Split it into features, each
+  its own branch and MR.
+- **Not a second trunk.** Every repo branches off `main`. There is no longer an `initialization`
+  trunk.
+
+## How it is checked
+
+The end state is visible, and its absence is the smell:
+
+- After a feature merges, `git branch -r | grep feat/<slug>` and `git worktree list` return
+  nothing for it. A surviving branch or worktree means step 6 was skipped.
+- More than one open MR in a repo that share no feature name, or a stack of MRs none of which is
+  merged, is the failure this playbook exists to prevent — stop and consolidate before opening
+  more.