[claude] Enforce one-agent-per-issue via labels, document auto-delete branches (#1220) (#1222)

AGENTS.md

@@ -34,6 +34,44 @@ Read [`CLAUDE.md`](CLAUDE.md) for architecture patterns and conventions.
 
 ---
 
+## One-Agent-Per-Issue Convention
+
+**An issue must only be worked by one agent at a time.** Duplicate branches from
+multiple agents on the same issue cause merge conflicts, redundant code, and wasted compute.
+
+### Labels
+
+When an agent picks up an issue, add the corresponding label:
+
+| Label | Meaning |
+|-------|---------|
+| `assigned-claude` | Claude is actively working this issue |
+| `assigned-gemini` | Gemini is actively working this issue |
+| `assigned-kimi` | Kimi is actively working this issue |
+| `assigned-manus` | Manus is actively working this issue |
+
+### Rules
+
+1. **Before starting an issue**, check that none of the `assigned-*` labels are present.
+   If one is, skip the issue — another agent owns it.
+2. **When you start**, add the label matching your agent (e.g. `assigned-claude`).
+3. **When your PR is merged or closed**, remove the label (or it auto-clears when
+   the branch is deleted — see Auto-Delete below).
+4. **Never assign the same issue to two agents simultaneously.**
+
+### Auto-Delete Merged Branches
+
+`default_delete_branch_after_merge` is **enabled** on this repo. Branches are
+automatically deleted after a PR merges — no manual cleanup needed and no stale
+`claude/*`, `gemini/*`, or `kimi/*` branches accumulate.
+
+If you discover stale merged branches, they can be pruned with:
+```bash
+git fetch --prune
+```
+
+---
+
 ## Merge Policy (PR-Only)
 
 **Gitea branch protection is active on `main`.** This is not a suggestion.