Use threat-model.md in Phase 5 exploitation 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 5 consumption is intentionally deferred.

Phase 5 exploitation could use the threat model to keep exploit attempts realistic and scoped.

Proposal

Update Phase 5 prompts and supporting checks, where appropriate, so exploitation planning considers:

• attacker capabilities and explicit non-capabilities,
• documented exploit preconditions,
• trust boundaries crossed by the confirmed finding,
• affected assets and security objectives,
• existing controls that may narrow exploitability,
• open assumptions that would change exploit feasibility.

Acceptance criteria

• Phase 5 prompt explicitly references itemdb/notes/threat-model.md when present.
• Exploitation attempts do not assume capabilities contradicted by the threat model.
• Exploit documentation records material threat-model assumptions affecting feasibility.
• Existing Phase 5 behavior remains compatible with projects that do not yet have threat-model artifacts.