feat(ui): import comprehensive UX system from beads

Import beads' UX design system into gastown:

- Add internal/ui/ package with Ayu theme colors and semantic styling
  - styles.go: AdaptiveColor definitions for light/dark mode
  - terminal.go: TTY detection, NO_COLOR/CLICOLOR support
  - markdown.go: Glamour rendering with agent mode bypass
  - pager.go: Smart paging with GT_PAGER support

- Add colorized help output (internal/cmd/help.go)
  - Group headers in accent color
  - Command names styled for scannability
  - Flag types and defaults muted

- Add gt thanks command (internal/cmd/thanks.go)
  - Contributor display with same logic as bd thanks
  - Styled with Ayu theme colors

- Update gt doctor to match bd doctor UX
  - Category grouping (Core, Infrastructure, Rig, Patrol, etc.)
  - Semantic icons (✓ ⚠ ✖) with Ayu colors
  - Tree connectors for detail lines
  - Summary line with pass/warn/fail counts
  - Warnings section at end with numbered issues

- Migrate existing styles to use ui package
  - internal/style/style.go uses ui.ColorPass etc.
  - internal/tui/feed/styles.go uses ui package colors

Co-Authored-By: SageOx <ox@sageox.ai>

internal/ui/terminal.go

@@ -0,0 +1,63 @@
+package ui
+
+import (
+	"os"
+
+	"golang.org/x/term"
+)
+
+// IsTerminal returns true if stdout is connected to a terminal (TTY).
+func IsTerminal() bool {
+	return term.IsTerminal(int(os.Stdout.Fd()))
+}
+
+// ShouldUseColor determines if ANSI color codes should be used.
+// Respects NO_COLOR (https://no-color.org/), CLICOLOR, and CLICOLOR_FORCE conventions.
+func ShouldUseColor() bool {
+	// NO_COLOR takes precedence - any value disables color
+	if _, exists := os.LookupEnv("NO_COLOR"); exists {
+		return false
+	}
+
+	// CLICOLOR=0 disables color
+	if os.Getenv("CLICOLOR") == "0" {
+		return false
+	}
+
+	// CLICOLOR_FORCE enables color even in non-TTY
+	if _, exists := os.LookupEnv("CLICOLOR_FORCE"); exists {
+		return true
+	}
+
+	// default: use color only if stdout is a TTY
+	return IsTerminal()
+}
+
+// ShouldUseEmoji determines if emoji decorations should be used.
+// Disabled in non-TTY mode to keep output machine-readable.
+func ShouldUseEmoji() bool {
+	// GT_NO_EMOJI disables emoji output
+	if _, exists := os.LookupEnv("GT_NO_EMOJI"); exists {
+		return false
+	}
+
+	// default: use emoji only if stdout is a TTY
+	return IsTerminal()
+}
+
+// IsAgentMode returns true if the CLI is running in agent-optimized mode.
+// This is triggered by:
+//   - GT_AGENT_MODE=1 environment variable (explicit)
+//   - CLAUDE_CODE environment variable (auto-detect Claude Code)
+//
+// Agent mode provides ultra-compact output optimized for LLM context windows.
+func IsAgentMode() bool {
+	if os.Getenv("GT_AGENT_MODE") == "1" {
+		return true
+	}
+	// auto-detect Claude Code environment
+	if os.Getenv("CLAUDE_CODE") != "" {
+		return true
+	}
+	return false
+}