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

docs/pipeline.md

@@ -118,11 +118,21 @@ close`); no skill performs it automatically.
 
 ### brainstorm
 
-Optional discovery front-end. Gathers requirements, explores 2-3
-approaches with trade-offs, presents a sectioned design with user
-approval, then hands the ratified design to `specify` (it writes no
-spec itself). Skipped when the design is already settled in the
-sources — that work enters through `specify` directly.
+Optional discovery front-end. Enumerates the cycle's load-bearing design
+forks, then — instead of a serial interview — populates them with a parallel
+swarm of grounded, genuinely-opposed `synthetic-user` stances and triages the
+result: a fork the swarm settles on a *cited shared ground* was derivable and
+is decided (the user vetoes); a fork it splits on, or agrees on with no citable
+ground, is a direction fork the user settles. The orchestrator authors the 2-3
+approaches from the populated space (never the swarm) and hands the ratified
+design to `specify` (it writes no spec itself). A fire-check skips the swarm
+when there are too few forks or the project is below a grounding floor
+(greenfield, no ledger to cite against), falling back to a direct interview.
+Skipped entirely when the design is already settled in the sources — that work
+enters through `specify` directly. 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 to the
+user.
 
 ### specify