Swarm --as <soul>: persona per pane (exploratory)

Part of #16 and explicitly exploratory: the design note is unsure whether a persona without a task decomposition actually helps, or whether swarm is better left runner-only.

• thrawn swarm 42 43 --as builder injects the named soul into the swarm prompt via the #19 slot
• Trial it on a small batch and compare against a soulless leg before deciding whether it stays

Relates to #7 (interactive swarm sessions) in spirit: both are about giving a swarm pane more character than an argv template.

Agent brief

Safe to land live: the flag only affects runs that pass it. Add --as <soul> to the thrawn swarm argparse, resolve the named soul via the existing load_soul path (a per-invocation override beating the [personas] swarm mapping) and inject it through the existing {{persona}} slot in swarm.md. Unknown soul name dies with a clear message listing available souls. Add tests following the existing style in thrawn/tests/test_thrawn.py. Acceptance: make test and make lint green; behaviour without --as is byte-identical.

Start with the thrawn swarm argparse and the existing load_soul path, then inspect swarm.md and its {{persona}} slot. Follow the test style in thrawn/tests/test_thrawn.py for the named override and unknown-soul error. Done means make test and make lint pass, with behavior without --as remaining byte-identical.