feat: add agent schema validation with comprehensive testing (#774)

Introduce automated validation for agent YAML files using Zod to ensure
schema compliance across all agent definitions. This feature validates
17 agent files across core and module directories, catching structural
errors and maintaining consistency.

Schema Validation (tools/schema/agent.js):
- Zod-based schema validating metadata, persona, menu, prompts, and critical actions
- Module-aware validation: module field required for src/modules/**/agents/,
  optional for src/core/agents/
- Enforces kebab-case unique triggers and at least one command target per menu item
- Validates persona.principles as array (not string)
- Comprehensive refinements for data integrity

CLI Validator (tools/validate-agent-schema.js):
- Scans src/{core,modules/*}/agents/*.agent.yaml
- Parses with js-yaml and validates using Zod schema
- Reports detailed errors with file paths and field paths
- Exits 1 on failures, 0 on success
- Accepts optional project_root parameter for testing

Testing (679 lines across 3 test files):
- test/test-cli-integration.sh: CLI behavior and error handling tests
- test/unit-test-schema.js: Direct schema validation unit tests
- test/test-agent-schema.js: Comprehensive fixture-based tests
- 50 test fixtures covering valid and invalid scenarios
- ESLint configured to support CommonJS test files
- Prettier configured to ignore intentionally broken fixtures

CI Integration (.github/workflows/lint.yaml):
- Renamed from format-check.yaml to lint.yaml
- Added schema-validation job running npm run validate:schemas
- Runs in parallel with prettier and eslint jobs
- Validates on all pull requests

Data Cleanup:
- Fixed src/core/agents/bmad-master.agent.yaml: converted persona.principles
  from string to array format

Documentation:
- Updated schema-classification.md with validation section
- Documents validator usage, enforcement rules, and CI integration

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

Co-authored-by: Claude <noreply@anthropic.com>

test/fixtures/agent-schema/valid/menu-commands/all-command-types.agent.yaml

@@ -0,0 +1,39 @@
+# Test: Menu items with all valid command target types
+# Expected: PASS
+
+agent:
+  metadata:
+    id: all-commands
+    name: All Command Types
+    title: All Commands
+    icon: 🧪
+
+  persona:
+    role: Test agent with all command types
+    identity: I test all available command target types.
+    communication_style: Clear
+    principles:
+      - Test all command types
+
+  menu:
+    - trigger: workflow-test
+      description: Test workflow command
+      workflow: path/to/workflow
+    - trigger: validate-test
+      description: Test validate-workflow command
+      validate-workflow: path/to/validation
+    - trigger: exec-test
+      description: Test exec command
+      exec: npm test
+    - trigger: action-test
+      description: Test action command
+      action: perform_action
+    - trigger: tmpl-test
+      description: Test tmpl command
+      tmpl: path/to/template
+    - trigger: data-test
+      description: Test data command
+      data: path/to/data
+    - trigger: run-workflow-test
+      description: Test run-workflow command
+      run-workflow: path/to/workflow

test/fixtures/agent-schema/valid/menu-commands/multiple-commands.agent.yaml

@@ -0,0 +1,23 @@
+# Test: Menu item with multiple command targets
+# Expected: PASS - multiple targets are allowed
+
+agent:
+  metadata:
+    id: multiple-commands
+    name: Multiple Commands
+    title: Multiple Commands
+    icon: 🧪
+
+  persona:
+    role: Test agent with multiple command targets
+    identity: I test multiple command targets per menu item.
+    communication_style: Clear
+    principles:
+      - Test multiple targets
+
+  menu:
+    - trigger: multi-command
+      description: Menu item with multiple command targets
+      workflow: path/to/workflow
+      exec: npm test
+      action: perform_action