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>

docs/wisp-squash-design.md

@@ -272,7 +272,7 @@ Digests feed into the activity feed for observability:
 ```json
 {
   "type": "digest",
-  "agent": "gastown/witness",
+  "agent": "greenplace/witness",
   "timestamp": "2025-12-30T10:00:00Z",
   "summary": "Patrol cycle 47 complete",
   "metrics": {