feat(brainstorm): swarm-simulated discovery replacing the serial interview

Replace brainstorm's ~10 serial user round-trips with a parallel
populate-then-triage: enumerate the load-bearing forks, populate them with a
grounded synthetic-user swarm (mandatory per-answer citation), and triage — a
fork the swarm settles on a cited shared ground is decided (user vetoes), a
split or ungrounded-consensus is a direction fork the user settles. A
cross-check surfaces any fork the orchestrator's own enumeration held open but
the swarm closed. A fire-check + grounding floor skip the swarm when it would
be ceremony.

Under /boss the swarm runs autonomously: convergent forks are decided and
recorded on the reference issue, only a divergent direction fork (or a
sub-floor greenfield design) bounces. Retires trigger-4's "fresh brainstorm is
too high-context to run autonomously" rationale — the swarm is the compaction.

- brainstorm/agents/synthetic-user.md: new read-only swarm agent
- brainstorm/SKILL.md: full rewrite to the swarm flow + new Iron Law
- boss/SKILL.md: autonomous swarm-brainstorm (Iron Law, trigger 4, asymmetry,
  notifications, rationalisations, red flag)
- docs/pipeline.md: brainstorm phase description
- specify/SKILL.md: brainstorm characterised as swarm discovery, not interview

closes #19

specify/SKILL.md

@@ -24,7 +24,8 @@ spec. `specify` carries the universal invariant **no plan without an
 approved spec**.
 
 `specify` is the *producing* half of a deliberate split. `brainstorm`
-owns *deciding* — interview, approaches, ratification. `specify` owns
+owns *deciding* — discovery (a grounded synthetic-user swarm), approaches,
+ratification. `specify` owns
 *producing* — rendering that decision into the canonical artefact and
 clearing the gates. The split is the toolchain's deepest discipline
 applied one level up: `tdd → implement` and `debug → implement` split