docs: add .thf file format specification

[OELD]

Closes #33

Adds complete schema, example with annotation, and version compatibility notes.

[vercel]

@OELD is attempting to deploy a commit to the Exit Zero Labs Team on Vercel.

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[exitzerolabs-admin]

Thanks for picking this up — a canonical .thf spec is worth having, and surfacing it at docs/file-format.md instead of tucked under docs/knowledge/ is probably the right call long-term.

The blocker here is that the schema in this PR doesn't match the actual format. A few examples from the current Rust models and docs/knowledge/file-format.md:

• version is a string ("1.0"), not a number
• Top-level keys are elements, data_flows, trust_boundaries — not components, dataFlows, trustBoundary
• There's a required metadata block (title, author, created/modified) that's missing
• Threats reference elements/flows by ID (element:, flow:) and have mitigation.status + mitigation.description, not a flat mitigation string
• Severity is Low | Medium | High (capitalized)
• The diagrams section and inline layout fields (position, size, viewport) aren't documented

Easiest path forward: pull the schema + example straight from docs/knowledge/file-format.md (that's the source of truth, kept in sync with the serde models), restructure it into a proper spec format if you
want, and either replace the old file or turn it into a pointer. Happy to review once that's in.

[OELD]

Fixed, now using the actual schema from docs/knowledge/. Ready for re‑review.