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/feed.go

@@ -84,7 +84,7 @@ Examples:
   gt feed --plain               # Plain text output (bd activity)
   gt feed --window              # Open in dedicated tmux window
   gt feed --since 1h            # Events from last hour
-  gt feed --rig gastown         # Use gastown rig's beads`,
+  gt feed --rig greenplace         # Use gastown rig's beads`,
 	RunE: runFeed,
 }