Clarify convoy vs swarm terminology in docs and code

- Convoy: Persistent tracking unit for batched work across rigs
- Swarm: Ephemeral workers on a convoy (no separate tracking)

Changes:
- docs/convoy.md: New comprehensive convoy documentation
- docs/swarm.md: Updated to explain ephemeral nature
- docs/reference.md: Replace swarm section with convoy commands
- internal/cmd/convoy.go: Clarify help text
- internal/cmd/swarm.go: Mark as deprecated

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

docs/understanding-gas-town.md

@@ -49,7 +49,7 @@ Both do project work, but with key differences:
 
 **When to use Polecats**:
 - Discrete, well-defined tasks
-- Batch work (via swarms)
+- Batch work (tracked via convoys)
 - Parallelizable work
 - Work that benefits from supervision