feat(templates): Add local override support for role templates

Adds ability to override embedded role templates at town or rig level:
- Town: <townRoot>/templates/roles/<role>.md.tmpl
- Rig: <rigPath>/templates/roles/<role>.md.tmpl

Rig-level overrides take precedence over town-level.

This enables customizing polecat (or other role) behavior per-rig without
modifying gastown source, following the same 3-tier override pattern as
role configs.

New APIs:
- NewWithOverrides(townRoot, rigPath string) - loads templates with overrides
- HasRoleOverride(role string) bool - check if role has override
- RoleOverrideCount() int - count of loaded overrides

Implements sc-6ghhn

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

internal/cmd/prime_output.go

@@ -18,8 +18,14 @@ import (
 
 // outputPrimeContext outputs the role-specific context using templates or fallback.
 func outputPrimeContext(ctx RoleContext) error {
-	// Try to use templates first
-	tmpl, err := templates.New()
+	// Calculate rig path for template overrides
+	var rigPath string
+	if ctx.Rig != "" && ctx.TownRoot != "" {
+		rigPath = filepath.Join(ctx.TownRoot, ctx.Rig)
+	}
+
+	// Try to use templates with override support
+	tmpl, err := templates.NewWithOverrides(ctx.TownRoot, rigPath)
 	if err != nil {
 		// Fall back to hardcoded output if templates fail
 		return outputPrimeContextFallback(ctx)