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/tui/feed/styles.go

@@ -4,17 +4,18 @@ package feed
 import (
 	"github.com/charmbracelet/lipgloss"
 	"github.com/steveyegge/gastown/internal/constants"
+	"github.com/steveyegge/gastown/internal/ui"
 )
 
-// Color palette
+// Color palette using Ayu theme colors from ui package
 var (
-	colorPrimary   = lipgloss.Color("12")  // Blue
-	colorSuccess   = lipgloss.Color("10")  // Green
-	colorWarning   = lipgloss.Color("11")  // Yellow
-	colorError     = lipgloss.Color("9")   // Red
-	colorDim       = lipgloss.Color("8")   // Gray
-	colorHighlight = lipgloss.Color("14")  // Cyan
-	colorAccent    = lipgloss.Color("13")  // Magenta
+	colorPrimary   = ui.ColorAccent // Blue
+	colorSuccess   = ui.ColorPass   // Green
+	colorWarning   = ui.ColorWarn   // Yellow
+	colorError     = ui.ColorFail   // Red
+	colorDim       = ui.ColorMuted  // Gray
+	colorHighlight = lipgloss.AdaptiveColor{Light: "#59c2ff", Dark: "#59c2ff"} // Cyan (Ayu)
+	colorAccent    = lipgloss.AdaptiveColor{Light: "#d2a6ff", Dark: "#d2a6ff"} // Purple (Ayu)
 )
 
 // Styles for the feed TUI