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/session/manager.go

@@ -46,6 +46,13 @@ type StartOptions struct {
 
 	// Command overrides the default "claude" command.
 	Command string
+
+	// Account specifies the account handle to use (overrides default).
+	Account string
+
+	// ClaudeConfigDir is resolved CLAUDE_CONFIG_DIR for the account.
+	// If set, this is injected as an environment variable.
+	ClaudeConfigDir string
 }
 
 // Info contains information about a running session.
@@ -133,6 +140,11 @@ func (m *Manager) Start(polecat string, opts StartOptions) error {
 	_ = m.tmux.SetEnvironment(sessionID, "GT_RIG", m.rig.Name)
 	_ = m.tmux.SetEnvironment(sessionID, "GT_POLECAT", polecat)
 
+	// Set CLAUDE_CONFIG_DIR for account selection
+	if opts.ClaudeConfigDir != "" {
+		_ = m.tmux.SetEnvironment(sessionID, "CLAUDE_CONFIG_DIR", opts.ClaudeConfigDir)
+	}
+
 	// CRITICAL: Set beads environment for worktree polecats
 	// Polecats share the rig's beads directory (at rig root, not mayor/rig)
 	// BEADS_NO_DAEMON=1 prevents daemon from committing to wrong branch