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/doctor/town_git_check.go

@@ -20,6 +20,7 @@ func NewTownGitCheck() *TownGitCheck {
 		BaseCheck: BaseCheck{
 			CheckName:        "town-git",
 			CheckDescription: "Verify town root is under version control",
+			CheckCategory:    CategoryCore,
 		},
 	}
 }