feat: add agent schema validation with comprehensive testing

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/prompts/empty-prompts.agent.yaml

@@ -0,0 +1,23 @@
+# Test: Empty prompts array
+# Expected: PASS - empty array valid for optional field
+
+agent:
+  metadata:
+    id: empty-prompts
+    name: Empty Prompts
+    title: Empty Prompts
+    icon: 🧪
+
+  persona:
+    role: Test agent with empty prompts
+    identity: I am a test agent with empty prompts array.
+    communication_style: Clear
+    principles:
+      - Test empty arrays
+
+  prompts: []
+
+  menu:
+    - trigger: help
+      description: Show help
+      action: display_help

test/fixtures/agent-schema/valid/prompts/no-prompts.agent.yaml

@@ -0,0 +1,21 @@
+# Test: No prompts field (optional)
+# Expected: PASS
+
+agent:
+  metadata:
+    id: no-prompts
+    name: No Prompts
+    title: No Prompts
+    icon: 🧪
+
+  persona:
+    role: Test agent without prompts
+    identity: I am a test agent without prompts field.
+    communication_style: Clear
+    principles:
+      - Test optional fields
+
+  menu:
+    - trigger: help
+      description: Show help
+      action: display_help

test/fixtures/agent-schema/valid/prompts/valid-prompts-minimal.agent.yaml

@@ -0,0 +1,27 @@
+# Test: Prompts with required id and content only
+# Expected: PASS
+
+agent:
+  metadata:
+    id: valid-prompts-minimal
+    name: Valid Prompts Minimal
+    title: Valid Prompts
+    icon: 🧪
+
+  persona:
+    role: Test agent with minimal prompts
+    identity: I am a test agent with minimal prompt structure.
+    communication_style: Clear
+    principles:
+      - Test minimal prompts
+
+  prompts:
+    - id: prompt1
+      content: This is a valid prompt content
+    - id: prompt2
+      content: Another valid prompt
+
+  menu:
+    - trigger: help
+      description: Show help
+      action: display_help

test/fixtures/agent-schema/valid/prompts/valid-prompts-with-description.agent.yaml

@@ -0,0 +1,29 @@
+# Test: Prompts with optional description field
+# Expected: PASS
+
+agent:
+  metadata:
+    id: valid-prompts-description
+    name: Valid Prompts With Description
+    title: Valid Prompts Desc
+    icon: 🧪
+
+  persona:
+    role: Test agent with prompts including descriptions
+    identity: I am a test agent with complete prompt structure.
+    communication_style: Clear
+    principles:
+      - Test complete prompts
+
+  prompts:
+    - id: prompt1
+      content: This is a valid prompt content
+      description: This prompt does something useful
+    - id: prompt2
+      content: Another valid prompt
+      description: This prompt does something else
+
+  menu:
+    - trigger: help
+      description: Show help
+      action: display_help