feat: add template system for role contexts and messages

Implements gt-u1j.20: Prompt templates using go:embed.

- Add internal/templates package with embedded .md.tmpl files
- Role templates: mayor, witness, refinery, polecat, crew
- Message templates: spawn, nudge, escalation, handoff
- Update gt prime to use templates with fallback to hardcoded output
- Add crew role detection for <rig>/crew/<name>/ paths
- Include Gas Town architecture overview in all role contexts

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

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

internal/templates/messages/handoff.md.tmpl

@@ -0,0 +1,33 @@
+# 🤝 HANDOFF: {{ .Role }} Session
+
+## Current State
+
+**Role**: {{ .Role }}
+**Working on**: {{ .CurrentWork }}
+**Status**: {{ .Status }}
+
+{{ if .GitBranch }}
+**Git branch**: {{ .GitBranch }}
+{{ if .GitDirty }}⚠️ Working tree has uncommitted changes{{ end }}
+{{ end }}
+
+{{ if .PendingMail }}
+**Pending mail**: {{ .PendingMail }} messages in inbox
+{{ end }}
+
+## Next Steps
+
+{{ range $i, $step := .NextSteps }}
+{{ $i }}. {{ $step }}
+{{ end }}
+
+{{ if .Notes }}
+## Notes
+
+{{ .Notes }}
+{{ end }}
+
+---
+
+This handoff was generated automatically. Read the above carefully and continue
+where the previous session left off.