fix(mcp): type edit_frontmatter operations for OpenAI schema compatibility

[brndnblck]

Summary

Fixes #32.

OpenAI's function-calling API rejects edit_frontmatter at tool registration because the operations parameter schema has "items": true instead of a proper object schema. This happens because operations was typed as Vec<serde_json::Value>, and schemars maps serde_json::Value to the JSON Schema true (any value).

Changes

• Add FrontmatterOpKind enum with #[serde(rename_all = "snake_case")] and Deserialize + JsonSchema derives
• Add FrontmatterOpInput struct with typed op, key, value fields
• Update EditFrontmatterParams (MCP) and EditFrontmatterBody (HTTP) to use Vec<FrontmatterOpInput>
• Simplify parse_frontmatter_ops in both serve.rs and http.rs to match on the typed enum
• Add regression test asserting operations.items is an object schema (not true)
• Add deserialization round-trip test for all 6 variants
• Add test confirming unknown op variants are rejected at deserialization

Generated schema (after fix)

"operations": {
  "type": "array",
  "items": {
    "$ref": "#/$defs/FrontmatterOpInput"
  }
}

Where FrontmatterOpInput defines op as an enum with the 6 allowed values.

Wire format

Unchanged. {"op": "set", "key": "status", "value": "done"} works identically.

Testing

• cargo fmt --check -- clean
• cargo clippy -- -D warnings -- clean
• cargo test --lib -- 461 passed (458 existing + 3 new)