fix: use tag-specific complexity reports (#857)

* fix(expand-task): Use tag-specific complexity reports

- Add getTagAwareFilePath utility function to resolve tag-specific file paths
- Update expandTask to use tag-aware complexity report paths
- Fix issue where expand-task always used default complexity report
- Add comprehensive tests for getTagAwareFilePath utility
- Ensure proper handling of file extensions and directory structures

Fixes #850: Expanding tasks not using tag-specific complexity reports

The expandTask function now correctly uses complexity reports specific
to the current tag context (e.g., task-complexity-report_feature-branch.json)
instead of always using the default task-complexity-report.json file.

This enables proper task expansion behavior when working with multiple
tag contexts, ensuring complexity analysis is tag-specific and accurate.

* chore: Add changeset for tag-specific complexity reports fix

* test(expand-task): Add tests for tag-specific complexity report integration

- Introduced a new test suite for verifying the integration of tag-specific complexity reports in the expandTask function.
- Added a test case to ensure the correct complexity report is used when available for a specific tag.
- Mocked file system interactions to simulate the presence of tag-specific complexity reports.

This enhances the test coverage for task expansion behavior, ensuring it accurately reflects the complexity analysis based on the current tag context.

* refactor(task-manager): unify and simplify tag-aware file path logic and tests

- Reformatted imports and cleaned up comments in test files for readability
- Centralized mocks: moved getTagAwareFilePath & slugifyTagForFilePath
  mocks to setup.js for consistency and maintainability
- Simplified utils/getTagAwareFilePath: replaced manual parsing with
  path.parse() & path.format(); improved extension handling
- Enhanced test mocks for path.parse, path.format & reset path.join
  in beforeEach to avoid interference
- All tests now pass consistently; no change in functionality

tests/unit/scripts/modules/task-manager/expand-task.test.js

@@ -3,6 +3,10 @@
  */
 import { jest } from '@jest/globals';
 import fs from 'fs';
+import {
+	createGetTagAwareFilePathMock,
+	createSlugifyTagForFilePathMock
+} from './setup.js';
 
 // Mock the dependencies before importing the module under test
 jest.unstable_mockModule('../../../../../scripts/modules/utils.js', () => ({
@@ -36,6 +40,8 @@ jest.unstable_mockModule('../../../../../scripts/modules/utils.js', () => ({
 		}
 		return allTasks;
 	}),
+	getTagAwareFilePath: createGetTagAwareFilePathMock(),
+	slugifyTagForFilePath: createSlugifyTagForFilePathMock(),
 	readComplexityReport: jest.fn(),
 	markMigrationForNotice: jest.fn(),
 	performCompleteTagMigration: jest.fn(),
@@ -649,6 +655,61 @@ describe('expandTask', () => {
 		});
 	});
 
+	describe('Complexity Report Integration (Tag-Specific)', () => {
+		test('should use tag-specific complexity report when available', async () => {
+			// Arrange
+			const tasksPath = 'tasks/tasks.json';
+			const taskId = '1'; // Task in feature-branch
+			const context = {
+				mcpLog: createMcpLogMock(),
+				projectRoot: '/mock/project/root',
+				tag: 'feature-branch'
+			};
+
+			// Stub fs.existsSync to simulate complexity report exists for this tag
+			const existsSpy = jest
+				.spyOn(fs, 'existsSync')
+				.mockImplementation((filepath) =>
+					filepath.endsWith('task-complexity-report_feature-branch.json')
+				);
+
+			// Stub readJSON to return complexity report when reading the report path
+			readJSON.mockImplementation((filepath, projectRootParam, tagParam) => {
+				if (filepath.includes('task-complexity-report_feature-branch.json')) {
+					return {
+						complexityAnalysis: [
+							{
+								taskId: 1,
+								complexityScore: 8,
+								recommendedSubtasks: 5,
+								reasoning: 'Needs five detailed steps',
+								expansionPrompt: 'Please break this task into 5 parts'
+							}
+						]
+					};
+				}
+				// Default tasks data for tasks.json
+				const sampleTasksCopy = JSON.parse(JSON.stringify(sampleTasks));
+				const selectedTag = tagParam || 'master';
+				return {
+					...sampleTasksCopy[selectedTag],
+					tag: selectedTag,
+					_rawTaggedData: sampleTasksCopy
+				};
+			});
+
+			// Act
+			await expandTask(tasksPath, taskId, undefined, false, '', context, false);
+
+			// Assert - generateTextService called with systemPrompt for 5 subtasks
+			const callArg = generateTextService.mock.calls[0][0];
+			expect(callArg.systemPrompt).toContain('Generate exactly 5 subtasks');
+
+			// Clean up stub
+			existsSpy.mockRestore();
+		});
+	});
+
 	describe('Error Handling', () => {
 		test('should handle non-existent task ID', async () => {
 			// Arrange