fix(cli): quote YAML frontmatter scalar values

[chaliy]

Motivation

• Prevent frontmatter injection and parsing corruption by ensuring untrusted strings emitted in the markdown YAML frontmatter (request URL and response metadata) are safely quoted and escaped instead of concatenated raw.

Description

• Added a small yaml_quote helper that uses JSON string escaping to safely quote scalar values and returns a valid YAML string representation.
• Applied yaml_quote to the url, source_content_type, last_modified, and filename fields in the CLI formatter in crates/fetchkit-cli/src/main.rs and the MCP formatter in crates/fetchkit-cli/src/mcp.rs.
• Updated unit tests in crates/fetchkit-cli/src/main.rs to expect quoted scalars and added a regression test that verifies newline and YAML-special payloads remain quoted and do not inject frontmatter keys.
• Changes are minimal and preserve existing markdown-first output while hardening frontmatter integrity.

Testing

• Ran formatter: cargo fmt --all which completed successfully.
• Ran unit and integration tests for the CLI: cargo test -p fetchkit-cli and all tests passed (including the new regression test) with no failures.

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Codex Task

[Copilot]

Pull request overview

This PR hardens markdown YAML frontmatter generation in the CLI and MCP formatter by quoting untrusted scalar metadata before emitting it.

Changes:

• Added yaml_quote helpers using JSON string escaping.
• Applied quoting to URL, content type, last modified, and filename frontmatter fields.
• Updated CLI tests and added a regression test for newline/YAML-special scalar injection.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/fetchkit-cli/src/main.rs	Quotes frontmatter scalar metadata and updates unit coverage.
crates/fetchkit-cli/src/mcp.rs	Applies the same scalar quoting to MCP markdown output.
crates/fetchkit-cli/tests/cli_integration.rs	Updates expected CLI markdown output for quoted URLs.

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