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/nudge.go

@@ -33,8 +33,8 @@ Special targets:
   deacon    Maps to the Deacon session (gt-deacon)
 
 Examples:
-  gt nudge gastown/furiosa "Check your mail and start working"
-  gt nudge gastown/alpha "What's your status?"
+  gt nudge greenplace/furiosa "Check your mail and start working"
+  gt nudge greenplace/alpha "What's your status?"
   gt nudge deacon session-started`,
 	Args: cobra.ExactArgs(2),
 	RunE: runNudge,