feat(gateway): enable 2-devs-per-cell parallelism + split-before-claim sizing

- Intake / Main-PM / Cell-PM prompts: enumerate independently-shippable work
  units, inherit the breakdown down the chain, and dispatch independents in
  parallel (dependency order, never one-at-a-time).
- Raise the code-spine concurrency cap from 1 to 2 per parent (one per cell
  developer) so both devs build in parallel; keep the same-assignee guard and
  the planning/documentation cap at 1, plus the cross-team planning exemption.
- Split-before-claim: hard-block an egregiously-bundled code leaf at delegate
  time so the PM splits it before any dev claims it; nudge the moderate band
  in the delegate success envelope.

tests/unit/gateway/test_spine_cap_remediate.py

@@ -27,6 +27,7 @@ def test_spine_cap_remediate_forbids_task_type_workaround() -> None:
         new_type="code",
         sibling=sibling,
         sib_assignee="be-dev-1",
+        cap=2,
     )
     remediate = env.remediate or ""
     assert "DO NOT work around" in remediate, (
@@ -43,6 +44,7 @@ def test_spine_cap_remediate_warns_about_verification_subtasks() -> None:
         new_type="code",
         sibling=sibling,
         sib_assignee="be-dev-1",
+        cap=2,
     )
     remediate = env.remediate or ""
     # Anti-pattern names — must appear so the model pattern-matches its own behavior.
@@ -57,5 +59,6 @@ def test_spine_cap_envelope_is_invalid_state() -> None:
         new_type="code",
         sibling=sibling,
         sib_assignee="be-dev-1",
+        cap=2,
     )
     assert env.error == "invalid_state"