Add BD_AGENT_MODE for ultra-compact output

Adds agent-optimized output mode for `bd list` triggered by:
- BD_AGENT_MODE=1 environment variable (explicit)
- CLAUDE_CODE environment variable (auto-detect)

Agent mode provides:
- Ultra-compact format: just "ID: Title" per line
- Lower default limit (20 vs 50) for context efficiency
- No colors, no emojis, no pager
- Defaults to open/in_progress only (existing behavior)

(bd-x2ht)

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

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

internal/ui/styles_test.go

@@ -152,3 +152,26 @@ func TestRenderCommandAndCategoryAreUppercaseSafe(t *testing.T) {
 		t.Fatalf("command output missing text: %q", cmd)
 	}
 }
+
+func TestIsAgentMode(t *testing.T) {
+	// Test default (no env vars) - t.Setenv automatically restores after test
+	t.Setenv("BD_AGENT_MODE", "")
+	t.Setenv("CLAUDE_CODE", "")
+	if IsAgentMode() {
+		t.Fatal("expected false with no env vars")
+	}
+
+	// Test BD_AGENT_MODE=1
+	t.Setenv("BD_AGENT_MODE", "1")
+	t.Setenv("CLAUDE_CODE", "")
+	if !IsAgentMode() {
+		t.Fatal("expected true with BD_AGENT_MODE=1")
+	}
+
+	// Test CLAUDE_CODE auto-detection
+	t.Setenv("BD_AGENT_MODE", "")
+	t.Setenv("CLAUDE_CODE", "something")
+	if !IsAgentMode() {
+		t.Fatal("expected true with CLAUDE_CODE set")
+	}
+}