docs: Make documentation convoy-first (gt-yg8bs)

Update all documentation to present convoy as the primary unit of work tracking:

- README.md: Show convoy create before sling in Quick Start and Workflows
- reference.md: Reorder to show Convoy Management before Work Assignment
- understanding-gas-town.md: Add convoy to dispatch workflow example
- mayor.md.tmpl: Add convoy commands to Work Management section
- crew.md.tmpl: Include convoy in dispatch workflow table

Convoy is now the standard workflow - all slings should be part of a convoy.

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

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

docs/understanding-gas-town.md

@@ -138,7 +138,8 @@ For work that should be owned by the target rig:
 # Create issue in target rig
 bd create --prefix beads "Fix authentication bug"
 
-# Sling to target rig's workers
+# Create convoy and sling to target rig
+gt convoy create "Auth fix" bd-xyz
 gt sling bd-xyz beads
 ```