Replace 'gastown' with 'greenplace' in user-facing docs/examples

Using "greenplace" (The Green Place from Mad Max: Fury Road) as the
canonical example project/rig name in documentation and help text.
This provides a clearer distinction from the actual gastown repo name.

Changes:
- docs/*.md: Updated all example paths and commands
- internal/cmd/*.go: Updated help text examples
- internal/templates/: Updated example references
- Tests: Updated to use greenplace in example session names

Note: Import paths (github.com/steveyegge/gastown) and actual code
paths referencing the gastown repo structure are unchanged.

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

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

internal/cmd/broadcast.go

@@ -35,7 +35,7 @@ The message is sent as a nudge to each worker's Claude Code session.
 
 Examples:
   gt broadcast "Check your mail"
-  gt broadcast --rig gastown "New priority work available"
+  gt broadcast --rig greenplace "New priority work available"
   gt broadcast --all "System maintenance in 5 minutes"
   gt broadcast --dry-run "Test message"`,
 	Args: cobra.ExactArgs(1),