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/cmd/mayor.go

@@ -122,6 +122,10 @@ func startMayorSession(t *tmux.Tmux) error {
 	// Set environment
 	_ = t.SetEnvironment(MayorSessionName, "GT_ROLE", "mayor")
 
+	// Apply Mayor theme
+	theme := tmux.MayorTheme()
+	_ = t.ConfigureGasTownSession(MayorSessionName, theme, "", "Mayor", "coordinator")
+
 	// Launch Claude in a respawn loop - session survives restarts
 	// The startup hook handles 'gt prime' automatically
 	// Use SendKeysDelayed to allow shell initialization after NewSession