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/metadata/empty-module-name-in-path.agent.yaml

@@ -0,0 +1,23 @@
+# Test: Empty module name in path (src/modules//agents/)
+# Expected: PASS - treated as core agent (empty module normalizes to null)
+# Path context: src/modules//agents/test.agent.yaml
+
+agent:
+  metadata:
+    id: empty-module-path
+    name: Empty Module in Path
+    title: Empty Module Path
+    icon: 🧪
+    # No module field - path has empty module name, treated as core
+
+  persona:
+    role: Test agent for empty module name in path
+    identity: I test the edge case where module name in path is empty.
+    communication_style: Clear
+    principles:
+      - Test path parsing edge cases
+
+  menu:
+    - trigger: help
+      description: Show help
+      action: display_help

test/fixtures/agent-schema/valid/metadata/malformed-path-treated-as-core.agent.yaml

@@ -0,0 +1,23 @@
+# Test: Malformed module path (no slash after module name) treated as core
+# Expected: PASS - malformed path returns null, treated as core agent
+# Path context: src/modules/bmm
+
+agent:
+  metadata:
+    id: malformed-path
+    name: Malformed Path Test
+    title: Malformed Path
+    icon: 🧪
+    # No module field - will be treated as core since path parsing returns null
+
+  persona:
+    role: Test agent for malformed path edge case
+    identity: I test edge cases in path parsing.
+    communication_style: Clear
+    principles:
+      - Test edge case handling
+
+  menu:
+    - trigger: help
+      description: Show help
+      action: display_help

test/fixtures/agent-schema/valid/metadata/module-agent-correct.agent.yaml

@@ -0,0 +1,23 @@
+# Test: Valid module agent with correct module field
+# Expected: PASS
+# Path context: src/modules/bmm/agents/module-agent-correct.agent.yaml
+
+agent:
+  metadata:
+    id: bmm-test
+    name: BMM Test Agent
+    title: BMM Test
+    icon: 🧪
+    module: bmm
+
+  persona:
+    role: Test module agent
+    identity: I am a module-scoped test agent.
+    communication_style: Professional
+    principles:
+      - Test module validation
+
+  menu:
+    - trigger: help
+      description: Show help
+      action: display_help