feat: Add prefix-based routing support for beads (gt-hrgpg)

- Add internal/beads/routes.go with helpers to manage routes.jsonl
- Update gt rig add to auto-append routes for new rigs
- Add prefix-conflict check to gt doctor

bd already has prefix routing via routes.jsonl - this wires up
Gas Town to maintain routes when rigs are added and detect conflicts.

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

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

internal/beads/routes.go

@@ -0,0 +1,156 @@
+// Package beads provides routing helpers for prefix-based beads resolution.
+package beads
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// Route represents a prefix-to-path routing rule.
+// This mirrors the structure in bd's internal/routing package.
+type Route struct {
+	Prefix string `json:"prefix"` // Issue ID prefix (e.g., "gt-")
+	Path   string `json:"path"`   // Relative path to .beads directory from town root
+}
+
+// RoutesFileName is the name of the routes configuration file.
+const RoutesFileName = "routes.jsonl"
+
+// LoadRoutes loads routes from routes.jsonl in the given beads directory.
+// Returns an empty slice if the file doesn't exist.
+func LoadRoutes(beadsDir string) ([]Route, error) {
+	routesPath := filepath.Join(beadsDir, RoutesFileName)
+	file, err := os.Open(routesPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil // No routes file is not an error
+		}
+		return nil, err
+	}
+	defer file.Close()
+
+	var routes []Route
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue // Skip empty lines and comments
+		}
+
+		var route Route
+		if err := json.Unmarshal([]byte(line), &route); err != nil {
+			continue // Skip malformed lines
+		}
+		if route.Prefix != "" && route.Path != "" {
+			routes = append(routes, route)
+		}
+	}
+
+	return routes, scanner.Err()
+}
+
+// AppendRoute appends a route to routes.jsonl in the town's beads directory.
+// If the prefix already exists, it updates the path.
+func AppendRoute(townRoot string, route Route) error {
+	beadsDir := filepath.Join(townRoot, ".beads")
+
+	// Load existing routes
+	routes, err := LoadRoutes(beadsDir)
+	if err != nil {
+		return fmt.Errorf("loading routes: %w", err)
+	}
+
+	// Check if prefix already exists
+	found := false
+	for i, r := range routes {
+		if r.Prefix == route.Prefix {
+			routes[i].Path = route.Path
+			found = true
+			break
+		}
+	}
+
+	if !found {
+		routes = append(routes, route)
+	}
+
+	// Write back
+	return WriteRoutes(beadsDir, routes)
+}
+
+// RemoveRoute removes a route by prefix from routes.jsonl.
+func RemoveRoute(townRoot string, prefix string) error {
+	beadsDir := filepath.Join(townRoot, ".beads")
+
+	// Load existing routes
+	routes, err := LoadRoutes(beadsDir)
+	if err != nil {
+		return fmt.Errorf("loading routes: %w", err)
+	}
+
+	// Filter out the prefix
+	var filtered []Route
+	for _, r := range routes {
+		if r.Prefix != prefix {
+			filtered = append(filtered, r)
+		}
+	}
+
+	// Write back
+	return WriteRoutes(beadsDir, filtered)
+}
+
+// WriteRoutes writes routes to routes.jsonl, overwriting existing content.
+func WriteRoutes(beadsDir string, routes []Route) error {
+	routesPath := filepath.Join(beadsDir, RoutesFileName)
+
+	file, err := os.Create(routesPath)
+	if err != nil {
+		return fmt.Errorf("creating routes file: %w", err)
+	}
+	defer file.Close()
+
+	for _, r := range routes {
+		data, err := json.Marshal(r)
+		if err != nil {
+			return fmt.Errorf("marshaling route: %w", err)
+		}
+		if _, err := file.Write(data); err != nil {
+			return fmt.Errorf("writing route: %w", err)
+		}
+		if _, err := file.WriteString("\n"); err != nil {
+			return fmt.Errorf("writing newline: %w", err)
+		}
+	}
+
+	return nil
+}
+
+// FindConflictingPrefixes checks for duplicate prefixes in routes.
+// Returns a map of prefix -> list of paths that use it.
+func FindConflictingPrefixes(beadsDir string) (map[string][]string, error) {
+	routes, err := LoadRoutes(beadsDir)
+	if err != nil {
+		return nil, err
+	}
+
+	// Group by prefix
+	prefixPaths := make(map[string][]string)
+	for _, r := range routes {
+		prefixPaths[r.Prefix] = append(prefixPaths[r.Prefix], r.Path)
+	}
+
+	// Filter to only conflicts (more than one path per prefix)
+	conflicts := make(map[string][]string)
+	for prefix, paths := range prefixPaths {
+		if len(paths) > 1 {
+			conflicts[prefix] = paths
+		}
+	}
+
+	return conflicts, nil
+}