feat: verify externally signed governance artifacts with pinned keys

[ernestprovo23]

The Agent Trust Kernel activation APIs previously required callers to supply their own artifact signature verifier. This change adds an opt-in Ed25519 implementation that accepts only explicitly enrolled public keys and artifact-kind roles under an independently pinned root configuration.

trust prepare validates an existing policy, runtime, adapter or bundle draft and writes unsigned canonical review/signing bytes. trust verify checks the returned detached signature against the pinned roots. The signature frame binds format version, artifact kind, key identity and exact canonical payload; keys authorized for policy cannot silently attest runtime state.

The product contains no private-key loader, key generator or signing command. Signature verification remains separate from activation, freshness and action authorization. Existing artifact/dependency checks and the default evidence gate remain enforced; live guard integration and durable receipts remain dependencies of DSE-717 and DSE-1076.

Validation:

• 46 new SDK/CLI/integration tests, including real signatures through all four activation paths, role/key/payload substitution, root-pin changes, malformed inputs and dependency drift.
• 134 focused tests including the existing PDP/PEP/conformance suites.
• Independent security review APPROVE at 5f91bb31873fbff9f30e90bbf1261d05cdb8255c; symlink-loop error handling includes a real subprocess regression. Ruff, strict MkDocs and desktop/mobile guide QA passed. Final-head CI passed 1,321 tests (2 skipped), 88.80% coverage, and 47 deterministic fuzz tests. All checks are green, including SDK 1.x parity, real Sigstore signing/verification, conformance, hash-locked installation and secret scanning.
• Dependency lock retains cryptography 50.0.0; only its direct-dependency annotation changes.

Security-specific development authorized in the current owner session. Merge remains subject to final security review and the exact-head release receipt.