docs: publish threat model & trust boundaries (#118)

[sergeyenin]

Summary

Adds a published threat model (proof-bar area 3) so an enterprise security reviewer can map Talon's boundaries without contacting us.

• docs/reference/threat-model.md — STRIDE-style:
  • Mermaid data-flow diagram of the gateway path with the host/binary trust zone.
  • Six trust boundaries: caller↔gateway, gateway↔provider, gateway↔MCP tool, operator↔host/process, admin plane, and at-rest SQLite stores.
  • Assets table, per-boundary controls, and a STRIDE threats/mitigations table with explicit residual risk.
  • Key management: TALON_SIGNING_KEY / TALON_SECRETS_KEY / TALON_ADMIN_KEY — location, "set explicitly in production" (defaults are per-machine derived), rotation semantics (historical evidence verifies only under its original signing key), and blast radius.
  • HMAC does/does not prove (integrity under key custody, not correctness; symmetric, not third-party non-repudiation).

Cross-linked from SECURITY.md (expands the existing snapshot), the docs index (Reference + Proof Pack), and LIMITATIONS.md (no longer "forthcoming"). Claims grounded in internal/{gateway,evidence,secrets,policy,classifier}.

Closes #118.

Test plan

• Claim-discipline guard passes.
• All referenced source files and doc links verified to resolve.
• Docs workflow (link check + claim guard) passes in CI.
• Mermaid diagram renders on GitHub.

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Note

Low Risk
Documentation-only change; no runtime, auth, or data-path behavior is modified.

Overview
Publishes docs/reference/threat-model.md so reviewers can map Talon’s gateway attack surface without maintainer contact. The doc adds a Mermaid data-flow diagram, six trust boundaries, an assets table, per-boundary controls, a STRIDE threats/mitigations table (with residual risk), operator key guidance for TALON_SIGNING_KEY / TALON_SECRETS_KEY / TALON_ADMIN_KEY, and a clear HMAC does/does not prove section.

Cross-links replace “forthcoming” in LIMITATIONS.md with a link to the new doc, point SECURITY.md at the full model from the existing snapshot, and index the page under Reference and Proof Pack in docs/README.md.

^{Reviewed by Cursor Bugbot for commit b0bf9f3. Configure here.}