Add frontmatter crate for YAML frontmatter ser/de with serde integration

[devin-ai-integration]

Summary

Adds a new crates/frontmatter crate that provides a generic Document<T> type for parsing and serializing markdown files with YAML frontmatter. The crate supports native serde integration, allowing Document<T> to be embedded in other serde formats (JSON, YAML, etc.).

This consolidates the frontmatter handling pattern currently duplicated across granola, hooks, and deeplink2 plugins.

API:

// Parse markdown with frontmatter
let doc: Document<MyStruct> = Document::from_str(markdown)?;

// Serialize back to markdown
let markdown = doc.to_string()?;

// Works with HashMap for dynamic frontmatter
let doc: Document<HashMap<String, String>> = Document::from_str(markdown)?;

Key feature: YAML keys are automatically sorted alphabetically (recursively for nested structures) to ensure deterministic output regardless of insertion order.

Updates since last revision

• Added consistent alphabetical sorting for all YAML frontmatter keys
• Sorting is recursive (nested maps are also sorted)
• Added 2 new tests (test_sorted_keys, test_nested_sorted_keys) - now 12 tests total

Review & Testing Checklist for Human

• Verify struct field ordering is acceptable: The sorting converts to serde_yaml::Value before serializing, which means struct fields are also sorted alphabetically (e.g., {title, tags} becomes {tags, title}). If preserving struct field declaration order matters, this needs adjustment.
• Verify output format compatibility: The serialization outputs ---\n{yaml}---\n\n{content}. Existing code uses format!("---\n{}---\n", yaml) (single newline after closing delimiter). Confirm this difference is acceptable.
• Test roundtrip with real-world content: The parsing logic strips leading whitespace and handles newlines in specific ways. Verify content with multiple paragraphs, trailing newlines, etc. roundtrips correctly.
• Empty frontmatter edge case: When frontmatter is empty (---\n---), it parses "{}" which works for HashMap/Default types but may fail for required-field structs.

Suggested test plan:

1. Run cargo test -p frontmatter (12 tests pass)
2. Try parsing some existing exported markdown files from granola to verify compatibility
3. Verify the alphabetical key ordering meets your needs for diffs/version control

Notes

• This PR adds the crate but does not refactor existing consumers (granola, hooks, deeplink2) to use it - that can be a follow-up
• CRLF line endings and UTF-8 BOM are not explicitly handled

Link to Devin run: https://app.devin.ai/sessions/37d3adedfd034b8896917d3f15bda735
Requested by: yujonglee (@yujonglee)

[devin-ai-integration]

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