Add DOS / Resource Exhaustion as Threat 6 to security threat model

[north-echo]

Summary

Closes #121. Adds Denial of Service / Resource Exhaustion as Threat 6 to security-threat-model.md, based on the proposed text in the issue with the following adjustments (per my comment):

• Placement: New section is after the temporal split-payload cross-cutting pattern and before "Cross-cutting security principles," keeping numbered threats and cross-cutting patterns together.
• Priority list updated: DOS ranked docs: Add codebase context problem document and trim CLAUDE.md #3 between insider threat and agent drift, reflecting its high-impact/moderate-likelihood profile due to cost asymmetry and automated-response amplification.
• Cross-reference with Threat 2: Rate limiting discussion references Threat 2 rather than duplicating it, and clarifies the distinction (behavioral anomaly detection vs. volume-based resource protection).
• Scope trimmed: Defense considerations focus on what is unique to agentic DOS (cost budgets, loop circuit breakers, event debouncing, tiered trust). Standard infrastructure defenses (cgroups, Kata Containers) are acknowledged but kept brief.
• Open questions consolidated: Merged the overlapping rate-limit-configurability and burst-vs-attack questions into one, reducing from 6 to 5 focused questions.

Test plan

• Verify the document renders correctly on GitHub (headers, anchor links, markdown formatting)
• Confirm the Threat 2 cross-reference anchor link resolves correctly
• Review that the priority list numbering is consistent with the rest of the document

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

[ascerra]

This looks great! Thanks for your contribution!