Use threat-model.md in Phase 4 validation planning

[pruiz]

Context

The Phase 1 threat-modeling integration plan adds itemdb/notes/threat-model.md as a required Phase 1b artifact. The initial implementation will explicitly wire this artifact into Phase 2 and Phase 3, but Phase 4 consumption is intentionally deferred.

Phase 4 validation could benefit from the operational threat model when designing realistic validation attempts.

Proposal

Update Phase 4 prompts and gates, where appropriate, so validation planning considers:

• attacker capabilities and explicit non-capabilities,
• documented trust boundaries,
• affected assets and security objectives,
• existing controls that may block or narrow validation,
• open assumptions that change exploitability or validation strategy.

Acceptance criteria

• Phase 4 prompt explicitly references itemdb/notes/threat-model.md when present.
• Validation plans avoid assuming attacker capabilities contradicted by the threat model.
• Validation evidence or notes mention material threat-model assumptions when they affect the result.
• Existing Phase 4 behavior remains compatible with projects that do not yet have threat-model artifacts.