fix: use source language in context code fences

[spa5k]

Summary

• derive Markdown code fence language tags from each context code block's file extension
• keep recognized Rust fences while labeling TypeScript and Python snippets correctly
• default unknown extensions to an unlabeled fence

Fixes #143

Tests

• cargo test context::formatter::tests::
• cargo test --test graph_suite test_format_context_markdown

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Latest commit: abad447

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