feat: enforce declared agent review policy

[Alex-ghost599]

Problem

Agent callers need a machine-readable Review artifact and declared review-policy gate before Task mutation. The caller must never assert whether review was self or independent.

Changes

• build hash-bound ReviewV1 artifacts and refuse tampered Plans
• derive self/independent relationship only from declared reviewer/executor IDs
• provide bundled neutral and action-specific strict policies
• enforce hash, verdict, exact expiry, age, intended executor, warnings, actions, checked facts and assignee identifier types
• add non-interactive JSON review/schema commands with atomic file output
• return redacted JSON error envelopes with stable exit 2/8 and short stderr
• preserve the Issue bug: artifact validation context can bypass hash integrity #7 / PR fix: close artifact hash validation bypass #8 artifact-context security fix from current develop

Verification

• 78 tests passed after rebasing onto current develop
• Ruff format/check and strict mypy passed
• build/twine, pip-audit, privacy history scan and diff check passed
• independent validation-only review: Approved after closing 3 Important and 2 Minor findings

Clean-room and risk

• synthetic examples only; no profiles, tenant/member defaults, credentials or private-source identifiers
• declared agent identities are audit metadata, not cryptographic proof

Implements #6. The Issue will be explicitly closed after merge to develop.