docs: define Agent Trust Kernel contract (DSE-714)

[ernestprovo23]

Summary

• define the normative Agent Trust Kernel contract and twelve testable invariants
• inventory protected assets, authoritative identities, threat classes, fail-closed outcomes, and non-overridable critical classes
• bind DSE-715 through DSE-717 to the contract and add explicit conformance gates
• cross-link the contract from the documentation index, system context, and shipped threat models

Product boundary

This is a design contract for DSE-714. MCP-Warden v1.1 remains explicitly nonconformant until DSE-715 through DSE-717 implement and verify the required runtime behavior.

Adversarial review

• four Conclave adversarial passes, culminating in SHIP (08f29532471e43afbef843e2b4db8d48)
• final independent CSO review: SHIP
• all 15 binding findings resolved; no fail-open, authority, contradiction, or testability blocker remains

Verification

• Ruff: passed
• strict MkDocs build with CI-pinned Material 9.7.6: passed
• relative Markdown links: passed
• Mermaid rendering for the ATK contract and system context: passed
• deterministic property-fuzz suite (--hypothesis-seed=0): 35 passed
• repository suite: 603 passed, 2 skipped; the one sandbox-denied process-table test passed separately outside the sandbox (1 passed)
• git diff --check: passed
• required GitHub checks on commit 460c8c5: pending

Linear: DSE-714