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/markdown.go

@@ -0,0 +1,65 @@
+package ui
+
+import (
+	"os"
+
+	"github.com/charmbracelet/glamour"
+	"golang.org/x/term"
+)
+
+// RenderMarkdown renders markdown text with glamour styling.
+// Returns raw markdown on failure for graceful degradation.
+func RenderMarkdown(markdown string) string {
+	// agent mode outputs plain text for machine parsing
+	if IsAgentMode() {
+		return markdown
+	}
+
+	// no styling when colors are disabled
+	if !ShouldUseColor() {
+		return markdown
+	}
+
+	wrapWidth := getTerminalWidth()
+
+	renderer, err := glamour.NewTermRenderer(
+		glamour.WithAutoStyle(),
+		glamour.WithWordWrap(wrapWidth),
+	)
+	if err != nil {
+		return markdown
+	}
+
+	rendered, err := renderer.Render(markdown)
+	if err != nil {
+		return markdown
+	}
+
+	return rendered
+}
+
+// getTerminalWidth returns the terminal width for word wrapping.
+// Caps at 100 chars for readability (research suggests 50-75 optimal, 80-100 comfortable).
+// Falls back to 80 if detection fails.
+func getTerminalWidth() int {
+	const (
+		defaultWidth = 80
+		maxWidth     = 100
+	)
+
+	fd := int(os.Stdout.Fd())
+	if !term.IsTerminal(fd) {
+		return defaultWidth
+	}
+
+	width, _, err := term.GetSize(fd)
+	if err != nil || width <= 0 {
+		return defaultWidth
+	}
+
+	if width > maxWidth {
+		return maxWidth
+	}
+
+	return width
+}