Implement deterministic context filtering, retrieval and selection

[andrewperis]

Parent epic: #5

Summary

Implement deterministic context filtering, retrieval and selection for Context Builder v1.

Ownership and dependencies

• Repositories: platform.
• Depends on: Implement verified source adapters and context normalization #78.
• Baseline: released Prompt SDK v1.0.0 and Context Package v1.0.0; do not change immutable contract bytes.

Scope and deliverables

• Filter authorized normalized candidates by explicit prompt/build requirements, source version, validity, continuity/canon scope and classification before ranking.
• Support exact candidate selection and a documented deterministic lexical/reference ranking baseline; make score/tie-break rules and policy version explicit.
• Define deduplication, supersession/conflict handling, provenance-preserving excerpts and no-match behavior without inventing facts or silently preferring private material.
• Exclude hosted embeddings, paid retrieval and LLM summarization from the default v1 path; keep a documented extension boundary for later reviewed work.

Acceptance criteria

• Repeated runs and permuted source enumeration produce identical ordered selections under the same policy.
• Ineligible candidates never enter ranking/results, and contradictory or ambiguous authority fails or reports a documented explicit state.
• Required-slot no-match and optional omissions have distinct actionable diagnostics.
• Selections retain exact source/fragment evidence, with synthetic relevance and adversarial tests.

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.