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/witness.go

@@ -266,6 +266,10 @@ func runWitnessAttach(cmd *cobra.Command, args []string) error {
 		t.SetEnvironment(sessionName, "GT_ROLE", "witness")
 		t.SetEnvironment(sessionName, "GT_RIG", rigName)
 
+		// Apply theme (same as rig polecats)
+		theme := tmux.AssignTheme(rigName)
+		_ = t.ConfigureGasTownSession(sessionName, theme, rigName, "witness", "witness")
+
 		// Launch Claude in a respawn loop
 		loopCmd := `while true; do echo "👁️ Starting Witness for ` + rigName + `..."; claude --dangerously-skip-permissions; echo ""; echo "Witness exited. Restarting in 2s... (Ctrl-C to stop)"; sleep 2; done`
 		if err := t.SendKeysDelayed(sessionName, loopCmd, 200); err != nil {