feat: auto-detect non-TTY and adjust output (bd-xrwy)

Add TTY detection to automatically disable ANSI colors when stdout is
piped or redirected. Respects standard conventions:
- NO_COLOR environment variable (no-color.org)
- CLICOLOR=0 disables color
- CLICOLOR_FORCE enables color even in non-TTY

Also adds ShouldUseEmoji() helper controlled by BD_NO_EMOJI.
Pager already disabled non-TTY in previous work (bd-jdz3).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/ui/terminal.go

@@ -0,0 +1,52 @@
+// Package ui provides terminal styling and output helpers for beads CLI.
+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 standard conventions:
+//   - NO_COLOR: https://no-color.org/ - disables color if set
+//   - CLICOLOR=0: disables color
+//   - CLICOLOR_FORCE: forces color even in non-TTY
+//   - Falls back to TTY detection
+func ShouldUseColor() bool {
+	// NO_COLOR standard - any value disables color
+	if os.Getenv("NO_COLOR") != "" {
+		return false
+	}
+
+	// CLICOLOR=0 disables color
+	if os.Getenv("CLICOLOR") == "0" {
+		return false
+	}
+
+	// CLICOLOR_FORCE forces color even in non-TTY
+	if os.Getenv("CLICOLOR_FORCE") != "" {
+		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.
+// Can be controlled with BD_NO_EMOJI environment variable.
+func ShouldUseEmoji() bool {
+	// Explicit disable
+	if os.Getenv("BD_NO_EMOJI") != "" {
+		return false
+	}
+
+	// Default: use emoji only if stdout is a TTY
+	return IsTerminal()
+}