Implement a read-only handoff validator

[dapi]

Parent: #104 — Add durable orchestration and Task Capsule model for Memory Bank

Goal

Provide a reproducible validator that determines whether a task may transition to its next role or stage.

Scope

• Validate Task Capsule structure and required fields.
• Validate artifact existence, frontmatter, status, and upstream owner links.
• Validate required evidence and traceability references.
• Validate that the transition is allowed by the selected workflow.
• Return structured diagnostics with failure reason and next action.
• Add focused fixtures or tests for valid, invalid, and blocked handoffs.

Out of scope

• Mutating task documents automatically.
• Starting agents or workflows.
• Runtime scheduling or retries.

Acceptance criteria

• Validator has a documented command/interface.
• Valid handoff returns success.
• Missing artifact, invalid status, broken ownership, and missing evidence are reported separately.
• Diagnostics are actionable for a follow-up model session.
• Validator is read-only and covered by tests.
• Existing template lint/doctor checks remain green.

Dependencies

• Depends on Task Capsule contract and role/handoff protocol subissues.

References

• Article
• Task Routing
• Feature Flow
• Memory Bank CLI docs