Implement budgeted Context Package v1 assembly

[andrewperis]

Parent epic: #5

Summary

Implement budgeted Context Package v1 assembly for Context Builder v1.

Ownership and dependencies

• Repositories: platform.
• Depends on: Implement deterministic context filtering, retrieval and selection #79.
• Baseline: released Prompt SDK v1.0.0 and Context Package v1.0.0; do not change immutable contract bytes.

Scope and deliverables

• Bind selected candidates to declared prompt slots and construct valid Context Package v1 sections, sources, package/builder identity and canonical manifest identity.
• Enforce per-slot and total byte budgets plus explicitly configured token estimates where supported; record tokenizer/version and never claim byte counts are exact model tokens.
• Use deterministic selection/omission rules, preserve JSON validity and UTF-8 boundaries, and fail when required context cannot fit instead of silently truncating meaning.
• Propagate maximum classification, provenance, purpose, creation/review/expiry limits and source constraints without minting use authorization.

Acceptance criteria

• Produced packages pass published Context Package schema and semantic validation.
• Budget boundary tests cover multibyte text, JSON, required/optional slots, zero capacity and oversized candidates.
• Byte/digest/source links and total_content_bytes verify exactly; ordering is reproducible.
• No generated package grants itself authority, lowers classification or exceeds source validity.

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.