feat(tmux): add per-rig color themes and dynamic status line (gt-vc1n)

Add tmux status bar theming for Gas Town sessions:

- Per-rig color themes auto-assigned via consistent hashing
- 10 curated dark themes (ocean, forest, rust, plum, etc.)
- Special gold/dark theme for Mayor
- Dynamic status line showing current issue and mail count
- Mayor status shows polecat/rig counts

New commands:
- gt theme --list: show available themes
- gt theme apply: apply to running sessions
- gt issue set/clear: agents update their current issue
- gt status-line: internal command for tmux refresh

Status bar format:
- Left: [rig/worker] role
- Right: <issue> | <mail> | HH:MM

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

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

internal/session/manager.go

@@ -132,6 +132,10 @@ func (m *Manager) Start(polecat string, opts StartOptions) error {
 	_ = m.tmux.SetEnvironment(sessionID, "BEADS_NO_DAEMON", "1")
 	_ = m.tmux.SetEnvironment(sessionID, "BEADS_AGENT_NAME", fmt.Sprintf("%s/%s", m.rig.Name, polecat))
 
+	// Apply theme
+	theme := tmux.AssignTheme(m.rig.Name)
+	_ = m.tmux.ConfigureGasTownSession(sessionID, theme, m.rig.Name, polecat, "polecat")
+
 	// Send initial command
 	command := opts.Command
 	if command == "" {