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/templates.go

@@ -0,0 +1,128 @@
+// Package templates provides embedded templates for role contexts and messages.
+package templates
+
+import (
+	"bytes"
+	"embed"
+	"fmt"
+	"text/template"
+)
+
+//go:embed roles/*.md.tmpl messages/*.md.tmpl
+var templateFS embed.FS
+
+// Templates manages role and message templates.
+type Templates struct {
+	roleTemplates    *template.Template
+	messageTemplates *template.Template
+}
+
+// RoleData contains information for rendering role contexts.
+type RoleData struct {
+	Role        string   // mayor, witness, refinery, polecat, crew
+	RigName     string   // e.g., "gastown"
+	TownRoot    string   // e.g., "/Users/steve/ai"
+	WorkDir     string   // current working directory
+	Polecat     string   // polecat name (for polecat role)
+	Polecats    []string // list of polecats (for witness role)
+	BeadsDir    string   // BEADS_DIR path
+	IssuePrefix string   // beads issue prefix
+}
+
+// SpawnData contains information for spawn assignment messages.
+type SpawnData struct {
+	Issue       string
+	Title       string
+	Priority    int
+	Description string
+	Branch      string
+	RigName     string
+	Polecat     string
+}
+
+// NudgeData contains information for nudge messages.
+type NudgeData struct {
+	Polecat    string
+	Reason     string
+	NudgeCount int
+	MaxNudges  int
+	Issue      string
+	Status     string
+}
+
+// EscalationData contains information for escalation messages.
+type EscalationData struct {
+	Polecat     string
+	Issue       string
+	Reason      string
+	NudgeCount  int
+	LastStatus  string
+	Suggestions []string
+}
+
+// HandoffData contains information for session handoff messages.
+type HandoffData struct {
+	Role         string
+	CurrentWork  string
+	Status       string
+	NextSteps    []string
+	Notes        string
+	PendingMail  int
+	GitBranch    string
+	GitDirty     bool
+}
+
+// New creates a new Templates instance.
+func New() (*Templates, error) {
+	t := &Templates{}
+
+	// Parse role templates
+	roleTempl, err := template.ParseFS(templateFS, "roles/*.md.tmpl")
+	if err != nil {
+		return nil, fmt.Errorf("parsing role templates: %w", err)
+	}
+	t.roleTemplates = roleTempl
+
+	// Parse message templates
+	msgTempl, err := template.ParseFS(templateFS, "messages/*.md.tmpl")
+	if err != nil {
+		return nil, fmt.Errorf("parsing message templates: %w", err)
+	}
+	t.messageTemplates = msgTempl
+
+	return t, nil
+}
+
+// RenderRole renders a role context template.
+func (t *Templates) RenderRole(role string, data RoleData) (string, error) {
+	templateName := role + ".md.tmpl"
+
+	var buf bytes.Buffer
+	if err := t.roleTemplates.ExecuteTemplate(&buf, templateName, data); err != nil {
+		return "", fmt.Errorf("rendering role template %s: %w", templateName, err)
+	}
+
+	return buf.String(), nil
+}
+
+// RenderMessage renders a message template.
+func (t *Templates) RenderMessage(name string, data interface{}) (string, error) {
+	templateName := name + ".md.tmpl"
+
+	var buf bytes.Buffer
+	if err := t.messageTemplates.ExecuteTemplate(&buf, templateName, data); err != nil {
+		return "", fmt.Errorf("rendering message template %s: %w", templateName, err)
+	}
+
+	return buf.String(), nil
+}
+
+// RoleNames returns the list of available role templates.
+func (t *Templates) RoleNames() []string {
+	return []string{"mayor", "witness", "refinery", "polecat", "crew"}
+}
+
+// MessageNames returns the list of available message templates.
+func (t *Templates) MessageNames() []string {
+	return []string{"spawn", "nudge", "escalation", "handoff"}
+}