feat: expand frontmatter_validate to full vault schema validation

[maehwasoo]

Problem statement

frontmatter_validate currently focuses on required-key presence and id/created consistency, plus typed-link diagnostics. This leaves a validation gap for full vault schema rules (field types, enum-allowed values, and schema-level constraints), so invalid metadata can pass validation and later degrade retrieval/filter quality.

Proposed solution

Expand frontmatter_validate from minimal presence checks to full schema validation aligned with vault standards.

• Schema coverage:
  • Validate type expectations for core frontmatter fields (string/array/optional constraints).
  • Validate enum-allowed values for fields such as entity, layer, and status.
  • Keep existing typed-link diagnostics behavior and modes (off/warn/error) compatible.
• Diagnostics contract:
  • Return structured diagnostics with stable category/code/path/field information.
  • Preserve current summary fields while adding schema-specific counts/details.
  • Keep default mode safe for existing users (warn-first behavior unless explicitly set to error).
• Standards alignment:
  • Use docs/standards/vault/* as source-of-truth for rule definitions and wording.
  • Add/refresh docs so tool behavior and standards examples stay in sync.
• Validation and tests:
  • Add unit/integration tests for valid/invalid enum/type cases and mixed-note vault scans.
  • Add regression tests to ensure legacy notes with normalization rules are handled predictably.

Constraints / context (optional)

• No automatic write/fix behavior in this issue; validation output only.
• Avoid breaking existing MCP consumers that rely on current response fields.
• Keep performance acceptable for large vault scans.