fix: Make Mayor/Deacon session names include town name

Session names `gt-mayor` and `gt-deacon` were hardcoded, causing tmux
session name collisions when running multiple towns simultaneously.

Changed to `gt-{town}-mayor` and `gt-{town}-deacon` format (e.g.,
`gt-ai-mayor`) to allow concurrent multi-town operation.

Key changes:
- session.MayorSessionName() and DeaconSessionName() now take townName param
- Added workspace.GetTownName() helper to load town name from config
- Updated all callers in cmd/, daemon/, doctor/, mail/, rig/, templates/
- Updated tests with new session name format
- Bead IDs remain unchanged (already scoped by .beads/ directory)

Fixes #60

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

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

internal/workspace/find.go

@@ -6,6 +6,8 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+
+	"github.com/steveyegge/gastown/internal/config"
 )
 
 // ErrNotFound indicates no workspace was found.
@@ -126,3 +128,35 @@ func IsWorkspace(dir string) (bool, error) {
 
 	return false, nil
 }
+
+// GetTownName loads the town name from the workspace's town.json config.
+// This is used for generating unique tmux session names that avoid collisions
+// when running multiple Gas Town instances.
+func GetTownName(townRoot string) (string, error) {
+	townConfigPath := filepath.Join(townRoot, PrimaryMarker)
+	townConfig, err := config.LoadTownConfig(townConfigPath)
+	if err != nil {
+		return "", fmt.Errorf("loading town config: %w", err)
+	}
+	return townConfig.Name, nil
+}
+
+// GetTownNameFromCwd locates the town root from the current working directory
+// and returns the town name from its configuration.
+func GetTownNameFromCwd() (string, error) {
+	townRoot, err := FindFromCwdOrError()
+	if err != nil {
+		return "", err
+	}
+	return GetTownName(townRoot)
+}
+
+// MustGetTownName returns the town name or panics if it cannot be loaded.
+// Use sparingly - prefer GetTownName with proper error handling.
+func MustGetTownName(townRoot string) string {
+	name, err := GetTownName(townRoot)
+	if err != nil {
+		panic(fmt.Sprintf("failed to get town name: %v", err))
+	}
+	return name
+}