Validate upstream readiness before requirement evidence import

[djm81]

Problem

The import-first runtime shipped by #168 accepts a pristine native Spec Kit feature template as requirement evidence. With the official specify 0.12.15 scaffold, placeholder Functional Requirements create six bogus speckit: records. OpenSpec changes also need a deterministic upstream-readiness check before imported records are treated as evidence.

Proposed scope

• Surface a shared core source-readiness result before the Requirements module persists imported OpenSpec or Spec Kit records.
• Reject incomplete native sources with zero persisted records and machine-readable diagnostics:
  • OpenSpec source fails openspec validate --strict --json when upstream validation is required.
  • Spec Kit source retains known official template placeholders, NEEDS CLARIFICATION markers, no substantive functional requirement, or no meaningful acceptance scenario where user stories are present.
• Preserve read-only behavior toward upstream source directories.
• Keep standard completed native OpenSpec and Spec Kit imports, stable IDs, content-hash provenance, and idempotency unchanged.
• Keep basic import portable: require the OpenSpec CLI only under an explicit or strict/enterprise upstream-validation policy; report upstream-validator-unavailable rather than silently claiming validation.

Ownership and dependency

This modules change only exposes the resulting core diagnostics and command behavior. Deterministic native parsing, placeholder detection, and upstream-validation policy belong to paired core issue nold-ai/specfact-cli#648 (follow-up to #350). The modules implementation is blocked until that core contract is released.

Acceptance evidence

• A pristine specify 0.12.15 generated feature imports zero records and returns a non-zero result with an incomplete-source diagnostic.
• A completed native Spec Kit feature imports normalized evidence without diagnostics.
• An invalid OpenSpec source imports zero records when upstream validation is required.
• Existing valid OpenSpec and Spec Kit imports remain read-only and idempotent.

OpenSpec change: requirements-04-upstream-source-readiness
Parent feature: #161
Follow-up to: #168
Core blocker: nold-ai/specfact-cli#648