feat(theme): add dark mode CLI theme support (#911)

Add the ability to force dark or light mode colors for CLI output,
overriding the automatic terminal background detection.

Changes:
- Add CLITheme field to TownSettings for persisting preference
- Add GetThemeMode() and HasDarkBackground() to ui package for
  theme detection with GT_THEME env var override
- Add ApplyThemeMode() to explicitly set lipgloss dark background
- Add 'gt theme cli' subcommand to view/set CLI theme preference
- Initialize theme in CLI startup (persistentPreRun)
- Add comprehensive tests for theme mode functionality

Usage:
- gt theme cli              # show current theme
- gt theme cli dark         # force dark mode
- gt theme cli light        # force light mode
- gt theme cli auto         # auto-detect (default)
- GT_THEME=dark gt status   # per-command override

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/root.go

@@ -8,7 +8,9 @@ import (
 	"strings"
 
 	"github.com/spf13/cobra"
+	"github.com/steveyegge/gastown/internal/config"
 	"github.com/steveyegge/gastown/internal/style"
+	"github.com/steveyegge/gastown/internal/ui"
 	"github.com/steveyegge/gastown/internal/version"
 	"github.com/steveyegge/gastown/internal/workspace"
 )
@@ -66,6 +68,9 @@ var branchCheckExemptCommands = map[string]bool{
 
 // persistentPreRun runs before every command.
 func persistentPreRun(cmd *cobra.Command, args []string) error {
+	// Initialize CLI theme (dark/light mode support)
+	initCLITheme()
+
 	// Get the root command name being run
 	cmdName := cmd.Name()
 
@@ -88,6 +93,22 @@ func persistentPreRun(cmd *cobra.Command, args []string) error {
 	return CheckBeadsVersion()
 }
 
+// initCLITheme initializes the CLI color theme based on settings and environment.
+func initCLITheme() {
+	// Try to load town settings for CLITheme config
+	var configTheme string
+	if townRoot, err := workspace.FindFromCwd(); err == nil && townRoot != "" {
+		settingsPath := config.TownSettingsPath(townRoot)
+		if settings, err := config.LoadOrCreateTownSettings(settingsPath); err == nil {
+			configTheme = settings.CLITheme
+		}
+	}
+
+	// Initialize theme with config value (env var takes precedence inside InitTheme)
+	ui.InitTheme(configTheme)
+	ui.ApplyThemeMode()
+}
+
 // warnIfTownRootOffMain prints a warning if the town root is not on main branch.
 // This is a non-blocking warning to help catch accidental branch switches.
 func warnIfTownRootOffMain() {