feat: implement account management for multi-account Claude Code (gt-3133)

Adds support for managing multiple Claude Code accounts in Gas Town:

- accounts.json config parsing in mayor/ directory
- gt account list/add/default commands
- GT_ACCOUNT env var support with priority resolution
- --account flag on gt spawn and gt crew at commands
- CLAUDE_CONFIG_DIR injection into tmux sessions

Priority order: GT_ACCOUNT env var > --account flag > default from config

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

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

internal/constants/constants.go

@@ -48,6 +48,9 @@ const (
 
 	// FileConfigYAML is the beads config file.
 	FileConfigYAML = "config.yaml"
+
+	// FileAccountsJSON is the accounts configuration file in mayor/.
+	FileAccountsJSON = "accounts.json"
 )
 
 // Git branch names.
@@ -154,3 +157,8 @@ func RigRuntimePath(rigPath string) string {
 func RigSettingsPath(rigPath string) string {
 	return rigPath + "/" + DirSettings
 }
+
+// MayorAccountsPath returns the path to mayor/accounts.json within a town root.
+func MayorAccountsPath(townRoot string) string {
+	return townRoot + "/" + DirMayor + "/" + FileAccountsJSON
+}