Implement context artifact lifecycle, safe caching and reproducible replay

[andrewperis]

Parent epic: #5

Summary

Implement context artifact lifecycle, safe caching and reproducible replay for Context Builder v1.

Ownership and dependencies

• Repositories: platform.
• Depends on: Implement budgeted Context Package v1 assembly #80.
• Baseline: released Prompt SDK v1.0.0 and Context Package v1.0.0; do not change immutable contract bytes.

Scope and deliverables

• Define immutable prepared artifact identities, serialization and explicit optional storage/cache interfaces; keep the default local/test flow memory-only.
• Key reuse by source identities, builder/policy versions, target, budgets and authorization scope; revalidate time/revocation before reuse.
• Separate retained artifact history from permission to reuse it; implement explicit expiry, eviction and access controls with secure-store adapters left injectable.
• Provide replay from approved pinned inputs and deterministic comparison, with no hidden wall clock, source refresh or access to revoked material.

Acceptance criteria

• Cache hits cannot cross caller/purpose/classification boundaries or bypass authorization checks.
• Expired/revoked inputs are not silently reused; historical evidence does not renew permission.
• Replay from identical authorized inputs preserves identity; changed policy/source/budget yields traceable differences.
• Retention/cleanup behavior is testable and protected bytes are never written to public source, fixtures or logs.

Implementation guardrails

Follow Constitution v1.0.0 at a9cc8a503aa30e17820edc62ac95f7cbe10e0564 and repository boundaries. Add focused tests with implementation; the suite task consolidates rather than postpones testing. Public issues, fixtures and docs use synthetic/already-public material only. Approved private inputs stay behind explicit secure artifact boundaries. No direct Lore checkout, silent declassification, self-approval or canon promotion.