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/unsling.go

@@ -23,8 +23,8 @@ another agent's hook.
 Examples:
   gt unsling                        # Clear my hook (whatever's there)
   gt unsling gt-abc                 # Only unsling if gt-abc is hooked
-  gt unsling gastown/joe            # Clear joe's hook
-  gt unsling gt-abc gastown/joe     # Unsling gt-abc from joe
+  gt unsling greenplace/joe            # Clear joe's hook
+  gt unsling gt-abc greenplace/joe     # Unsling gt-abc from joe
 
 The bead's status changes from 'pinned' back to 'open'.
 
@@ -154,7 +154,7 @@ func runUnsling(cmd *cobra.Command, args []string) error {
 // isAgentTarget checks if a string looks like an agent target rather than a bead ID.
 // Agent targets contain "/" or are known role names.
 func isAgentTarget(s string) bool {
-	// Contains "/" means it's a path like "gastown/joe"
+	// Contains "/" means it's a path like "greenplace/joe"
 	for _, c := range s {
 		if c == '/' {
 			return true