docs: clarify stream engine model, reduce swarm terminology (gt-wexr)

Gas Town is a stream engine, not a swarm engine. Polecats can start
and land independently at any time. "Swarm" is used lightly for
batch coordination, but there are no swarm IDs or batch boundaries.

Changes:
- architecture.md: Add stream engine intro paragraph, update
  references from "swarming" to "parallel polecats" or "batch"
- prompts.md: Rename SWARM_* messages to BATCH_*, update terminology
- crew.md.tmpl: Replace "swarm" references with "transient worker pool"
  and "batch work"

The gt swarm command and internal/swarm/ package remain as-is since
they provide the tooling for batch operations.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/templates/roles/crew.md.tmpl

@@ -13,7 +13,7 @@ You are a **crew worker** - the overseer's (human's) personal workspace within t
 - **Integrated**: Mail and handoff mechanics work just like other Gas Town agents
 
 **Key difference from polecats**: No one is watching you. You work directly with
-the overseer, not as part of a swarm.
+the overseer, not as part of a transient worker pool.
 
 ## Gas Town Architecture
 
@@ -127,7 +127,7 @@ If push fails (someone else pushed): `git pull --rebase && git push`
 - No automatic nudging if you seem stuck
 - No pre-kill verification checks
 - No escalation to Mayor if blocked
-- No automatic cleanup on swarm completion
+- No automatic cleanup when batch work completes
 
 **You are responsible for**:
 - Managing your own progress