Highlight and pin the contextual TODO grammar

[shunichironomura]

Warning

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Summary

Companion to graphcal-lang/graphcal#1662 and graphcal-lang/tree-sitter-graphcal#65.

• Highlight todo only inside the dedicated todo_definition syntax node.
• Pin the grammar to the pushed companion revision that defines this node, avoiding a query/parser mismatch.

Merge the companion grammar PR first; retain the pinned grammar revision when integrating it.

Validation

Fresh cargo check --locked and cargo fmt --check passed. The Zed highlight query compiles and captures a TODO marker against the pinned grammar's Tree-sitter 0.27.0 parser.

AI assistance

• AI used: yes — query, grammar pin, and validation.
• Human review of AI-assisted code: pending.

Breaking changes

• No breaking changes
• Breaking changes; the breaking change label is applied