Implement Jira issue timestamp comparison for sync (bd-0qx5)

Add actual timestamp comparison to detect conflicts during Jira sync:
- Add fetchJiraIssueTimestamp() to fetch a single issue's updated timestamp from Jira REST API
- Add extractJiraKey() to parse Jira issue key from external_ref URLs
- Add parseJiraTimestamp() to parse Jira's ISO 8601 timestamp format
- Update detectJiraConflicts() to fetch and compare Jira timestamps instead of marking all locally-updated issues as conflicts
- Update resolveConflictsByTimestamp() to show actual timestamp comparison results
- Update reimportConflicts() to display both local and Jira timestamps

Now only issues that have been modified on BOTH sides since the last sync are reported as conflicts, reducing false positives.

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

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

cmd/bd/jira_test.go

@@ -181,3 +181,114 @@ func TestPushStats(t *testing.T) {
 		t.Errorf("expected Errors to be 2, got %d", stats.Errors)
 	}
 }
+
+func TestExtractJiraKey(t *testing.T) {
+	tests := []struct {
+		name        string
+		externalRef string
+		want        string
+	}{
+		{
+			name:        "standard Jira Cloud URL",
+			externalRef: "https://company.atlassian.net/browse/PROJ-123",
+			want:        "PROJ-123",
+		},
+		{
+			name:        "Jira Server URL",
+			externalRef: "https://jira.company.com/browse/ISSUE-456",
+			want:        "ISSUE-456",
+		},
+		{
+			name:        "URL with trailing path",
+			externalRef: "https://company.atlassian.net/browse/ABC-789/some/path",
+			want:        "ABC-789/some/path",
+		},
+		{
+			name:        "no browse pattern",
+			externalRef: "https://github.com/org/repo/issues/123",
+			want:        "",
+		},
+		{
+			name:        "empty string",
+			externalRef: "",
+			want:        "",
+		},
+		{
+			name:        "only browse",
+			externalRef: "https://example.com/browse/",
+			want:        "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := extractJiraKey(tt.externalRef)
+			if got != tt.want {
+				t.Errorf("extractJiraKey(%q) = %q, want %q", tt.externalRef, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestParseJiraTimestamp(t *testing.T) {
+	tests := []struct {
+		name      string
+		timestamp string
+		wantErr   bool
+		wantYear  int
+	}{
+		{
+			name:      "standard Jira Cloud format with milliseconds",
+			timestamp: "2024-01-15T10:30:00.000+0000",
+			wantErr:   false,
+			wantYear:  2024,
+		},
+		{
+			name:      "Jira format with Z suffix",
+			timestamp: "2024-01-15T10:30:00.000Z",
+			wantErr:   false,
+			wantYear:  2024,
+		},
+		{
+			name:      "without milliseconds",
+			timestamp: "2024-01-15T10:30:00+0000",
+			wantErr:   false,
+			wantYear:  2024,
+		},
+		{
+			name:      "RFC3339 format",
+			timestamp: "2024-01-15T10:30:00Z",
+			wantErr:   false,
+			wantYear:  2024,
+		},
+		{
+			name:      "empty string",
+			timestamp: "",
+			wantErr:   true,
+		},
+		{
+			name:      "invalid format",
+			timestamp: "not-a-timestamp",
+			wantErr:   true,
+		},
+		{
+			name:      "with negative timezone offset",
+			timestamp: "2024-06-15T10:30:00.000-0500",
+			wantErr:   false,
+			wantYear:  2024,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := parseJiraTimestamp(tt.timestamp)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("parseJiraTimestamp(%q) error = %v, wantErr %v", tt.timestamp, err, tt.wantErr)
+				return
+			}
+			if !tt.wantErr && got.Year() != tt.wantYear {
+				t.Errorf("parseJiraTimestamp(%q) year = %d, want %d", tt.timestamp, got.Year(), tt.wantYear)
+			}
+		})
+	}
+}