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/refinery/manager.go

@@ -171,6 +171,10 @@ func (m *Manager) Start(foreground bool) error {
 	_ = t.SetEnvironment(sessionID, "GT_RIG", m.rig.Name)
 	_ = t.SetEnvironment(sessionID, "GT_REFINERY", "1")
 
+	// Apply theme (same as rig polecats)
+	theme := tmux.AssignTheme(m.rig.Name)
+	_ = t.ConfigureGasTownSession(sessionID, theme, m.rig.Name, "refinery", "refinery")
+
 	// Send the command to start refinery in foreground mode
 	// The foreground mode handles state updates and the processing loop
 	command := fmt.Sprintf("gt refinery start %s --foreground", m.rig.Name)