Add public threat model document (.github/THREAT_MODEL.md)

[dartpain]

Motivation

• Provide a centralized, public threat model for DocsGPT deployments to document risks and mitigations.
• Clarify assumptions, scope, and security objectives for open-source and self-hosted instances.
• Surface key attack surfaces and recommended baseline controls to guide secure deployments and future hardening.

Description

• Add a new file .github/THREAT_MODEL.md that defines the DocsGPT Public Threat Model and metadata such as classification and last-updated date.
• Document scope, trust boundaries, assets, main attack surfaces (auth, ingestion, SSRF, tool execution, frontend XSS, internal endpoints, DoS), and example attacker stories.
• Provide concrete mitigations and baseline controls including auth requirements, secret rotation, SSRF protections, parsing hardening, tool least-privilege, rate limiting, CSP, and monitoring.
• Include severity calibration, maintenance guidance, and references to OWASP resources and the repo SECURITY.md.

Testing

• No automated tests were added or executed as part of this documentation-only change.
• Existing CI/linting was not modified by this change and remains unchanged.
• Manual review of the document content was performed during authoring.

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Codex Task

[vercel]

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Project	Deployment	Actions	Updated (UTC)
nextra-docsgpt	Building	Preview, Comment	Apr 15, 2026 5:32pm
oss-docsgpt	Ready	Preview, Comment	Apr 15, 2026 5:32pm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.65%. Comparing base (aa938d7) to head (c18f85a).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2391   +/-   ##
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  Coverage   91.65%   91.65%           
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  Files         220      220           
  Lines       18372    18372           
=======================================
  Hits        16839    16839           
  Misses       1533     1533           

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