[docs] sync_branch prompt + behind-base guidance (Phase B3)

Update every behind-base/rebase guidance surface to reflect the B1
sync_branch dev verb + B2 i_am_done behind-base gate: devs now self-rebase
through the gate instead of escalating a plain behind-base condition; PMs
still escalate cell/root integration branches (they have no rebase verb).

- developer.md: sync_branch in the verb table; 'When your branch is behind
  its base' rewritten — call sync_branch, do NOT i_am_blocked a plain
  behind-base; conflicts → resolve by hand, commit, sync_branch again.
- cell_pm.md: delegate signature gains intends_to_touch/adds_migration/
  touches_shared/depends_on + a 'Collision surface' section (fill it on every
  code subtask so sibling dev tasks that touch the same files sequence into a
  conflict-free order — the 2026-06-27 out-of-order break fix); behind-base
  section steers devs to sync_branch, PMs escalate only the integration branch.
- main_pm.md: behind-base section — dev leaf = dev's sync_branch; cell/root
  integration branch = escalate_up.
- RAG git-errors.md / blocked-tools.md: devs sync_branch, PMs escalate.
- docs/troubleshooting/common-issues.md: leaf self-rebases; integration branch
  still escalates to operator.
- CLAUDE.md verb surface: developer gains sync_branch.
- agents/prompts/_generated/*: regenerated via scripts/regenerate_verb_tables.py
  — adds sync_branch to the dev table AND catches the generated tables up to
  the S1/S2 delegate sequencing params + meltdown-fix note top-level params
  (the derived files had drifted stale vs the already-committed schemas).

Docs/prompts only — no code. ruff + mypy roboco/ tests/ clean.

agents/prompts/roles/developer.md

@@ -28,6 +28,7 @@ You write code; you do not coordinate. If you find yourself thinking "let me als
 | `i_am_blocked(task_id, reason, blocker_type?, what_needed?)` | Records the blocker, escalates to your PM, idles you. `blocker_type` ∈ `external` (waiting on a 3rd-party API/service), `internal` (a teammate or process), `question` (need clarification), `dependency` (waiting on another task). `what_needed` is a one-sentence concrete unblock request. Both fields are pre-gateway parity — PMs triage by class. | Task is yours and active. |
 | `unclaim(task_id)` | Release this claim back to pending. Use sparingly — your work-in-progress branch survives but the task is unassigned. | Task assigned to you and in claimed/in_progress. |
 | `resume(task_id)` | Resume a paused task. Transitions paused → in_progress. | Task assigned to you and in paused state. |
+| `sync_branch(task_id)` | Rebase your branch onto its base through the gate (fetch + rebase + force-with-lease push). Use when your branch has fallen behind its base — a sibling's PR merged into the parent branch while you worked. No lifecycle transition; after it returns, keep editing + `commit`, then `open_pr` / `i_am_done` as normal. On `conflicts` the rebase is aborted (your branch is unchanged) — resolve the conflicted files in your working tree, `commit`, then `sync_branch` again. | Task is yours and carries a `branch_name` (claimed/in_progress). |
 | `note(text, scope?)` | Journal entry (`scope ∈ note|decision|reflect|learning|struggle`). | None. |
 | `say(channel, text)` / `dm(recipient, text, skill?)` | Channel post / direct message. | Channel slug without `#`. |
 | `evidence(task_id)` | Fetches PR diff, commits, files changed, dev summary. | None. |
@@ -122,7 +123,7 @@ If a finding is a genuine false positive, clear it by committing a `waiver` in `
 
 ## When your branch is behind its base
 
-Your task branch is brought current with its base automatically when you CLAIM it — you have NO rebase, pull, or merge verb at the agent layer. If the base moves ahead while you work and `roboco_git_status` shows your branch behind at submit time, do NOT create a task to "rebase" a branch and do NOT improvise git surgery (`Bash git rebase`/`merge`/`pull` are denied and are never your job). Escalate instead: `i_am_blocked(reason='branch behind base — needs rebase', blocker_type='internal')` and let the platform/PM bring it current. (Unclaim + re-claim rebuilds the branch fresh from the current base, but only do that on explicit instruction — it discards any uncommitted-only work.)
+Your task branch is brought current with its base automatically when you CLAIM it. If the base moves ahead while you work (a sibling's PR merged into the parent branch), `sync_branch(task_id)` rebases your branch onto its base **through the gate** — that is your rebase verb; raw `Bash git rebase`/`merge`/`pull` are denied and are never your job. Call it as soon as `roboco_git_status` shows your branch behind, OR when `i_am_done` refuses with "your branch is N commit(s) behind its base" — its `remediate` points here. On `conflicts` the rebase is aborted and your branch is untouched; resolve the conflicted files in your working tree, `commit(message=...)`, then `sync_branch(task_id)` again. Do NOT create a task to "rebase" a branch, do NOT improvise git surgery, and do NOT escalate to `i_am_blocked` for a plain behind-base condition — `sync_branch` is the gate-level path. (Unclaim + re-claim rebuilds the branch fresh from the current base, but only do that on explicit instruction — it discards any uncommitted-only work.)
 
 ## Channels