#167: Expose the passive auth surface tool

[justsml]

Stacked on #166 / PR #177.

Closes #167

Scope

• Add a thin Mastra tool over the secured stored-evidence passive-auth service.
• Accept only target ID, optional task ID, and bounded Artifact IDs from the model.
• Derive project and thread identity exclusively from trusted RequestContext.
• Return a compact canonical report summary without raw URLs or evidence text.
• Register the capability only for passive recon and composition stages as project-mutating, serialized, and non-idempotent.
• Invoke no HTTP, browser, shell, download, or active-probe capability.

Validation

• Real temporary-SQLite tool execution proves trusted attribution, durable Artifact/RAG creation, bounded output, and zero side effects without trusted context.
• Provider schema compatibility coverage passes.
• 12 focused integration tests, typecheck, targeted Biome, and diff checks pass.

Merge order

Merge PR #177 first, then retarget this PR to PR #177's base or main.

[justsml]

Council of Dans review: repair/defer. Highest-priority gate: Land after redaction/attribution and share the durable report-idempotency contract; trusted RequestContext remains the correct project/thread seam. Feature flags are not a substitute for authority, evidence-integrity, or durability fixes; use typed modes only where they provide a real rollout boundary.