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/spawn.md.tmpl

@@ -0,0 +1,33 @@
+# Work Assignment
+
+You have been assigned to work on the following issue:
+
+## Issue: {{ .Issue }}
+
+**Title**: {{ .Title }}
+**Priority**: P{{ .Priority }}
+**Branch**: {{ .Branch }}
+
+{{ if .Description }}
+## Description
+
+{{ .Description }}
+{{ end }}
+
+## Your Task
+
+1. Review the issue details with `bd show {{ .Issue }}`
+2. Work in your clone at `{{ .RigName }}/polecats/{{ .Polecat }}/`
+3. Commit changes regularly with clear messages
+4. When complete, run:
+   - `bd close {{ .Issue }}`
+   - `bd sync`
+   - `gt done`
+
+## Need Help?
+
+- File blocking issues: `bd create --title="Blocked: <reason>" --type=task`
+- Ask Witness: `gt mail send {{ .RigName }}/witness -s "Question" -m "..."`
+- Escalate: The Witness will escalate if you're stuck
+
+Good luck!