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/invalid/persona/empty-principles-array.agent.yaml

@@ -0,0 +1,23 @@
+# Test: Empty principles array
+# Expected: FAIL
+# Error code: too_small
+# Error path: agent.persona.principles
+# Error minimum: 1
+
+agent:
+  metadata:
+    id: empty-principles
+    name: Empty Principles
+    title: Empty Principles
+    icon: ❌
+
+  persona:
+    role: Test agent
+    identity: Test identity
+    communication_style: Test style
+    principles: []
+
+  menu:
+    - trigger: help
+      description: Show help
+      action: display_help

test/fixtures/agent-schema/invalid/persona/empty-string-in-principles.agent.yaml

@@ -0,0 +1,26 @@
+# Test: Empty string in principles array
+# Expected: FAIL
+# Error code: custom
+# Error path: agent.persona.principles[1]
+# Error message: agent.persona.principles[] must be a non-empty string
+
+agent:
+  metadata:
+    id: empty-principle-string
+    name: Empty Principle String
+    title: Empty Principle
+    icon: ❌
+
+  persona:
+    role: Test agent
+    identity: Test identity
+    communication_style: Test style
+    principles:
+      - Valid principle
+      - "   "
+      - Another valid principle
+
+  menu:
+    - trigger: help
+      description: Show help
+      action: display_help

test/fixtures/agent-schema/invalid/persona/extra-persona-fields.agent.yaml

@@ -0,0 +1,26 @@
+# Test: Extra unknown fields in persona
+# Expected: FAIL
+# Error code: unrecognized_keys
+# Error path: agent.persona
+# Error keys: ["extra_field", "another_extra"]
+
+agent:
+  metadata:
+    id: extra-persona-fields
+    name: Extra Persona Fields
+    title: Extra Persona
+    icon: ❌
+
+  persona:
+    role: Test agent
+    identity: Test identity
+    communication_style: Test style
+    principles:
+      - Test principle
+    extra_field: Not allowed
+    another_extra: Also invalid
+
+  menu:
+    - trigger: help
+      description: Show help
+      action: display_help

test/fixtures/agent-schema/invalid/persona/missing-role.agent.yaml

@@ -0,0 +1,23 @@
+# Test: Missing required persona.role field
+# Expected: FAIL
+# Error code: invalid_type
+# Error path: agent.persona.role
+# Error expected: string
+
+agent:
+  metadata:
+    id: missing-role
+    name: Missing Role
+    title: Missing Role
+    icon: ❌
+
+  persona:
+    identity: Test identity
+    communication_style: Test style
+    principles:
+      - Test principle
+
+  menu:
+    - trigger: help
+      description: Show help
+      action: display_help

test/fixtures/agent-schema/invalid/persona/principles-as-string.agent.yaml

@@ -0,0 +1,23 @@
+# Test: principles as string instead of array
+# Expected: FAIL
+# Error code: invalid_type
+# Error path: agent.persona.principles
+# Error expected: array
+
+agent:
+  metadata:
+    id: principles-string
+    name: Principles String
+    title: Principles String
+    icon: ❌
+
+  persona:
+    role: Test agent
+    identity: Test identity
+    communication_style: Test style
+    principles: This should be an array, not a string
+
+  menu:
+    - trigger: help
+      description: Show help
+      action: display_help