Add optional explanation providers over the canonical DiffGraph artifact

[nia-sg-bot]

Outcome

Preserve the useful provider-selection and privacy-consent intent from #12 without allowing providers to become competing topology engines.

Dependency gate

Do not implement this until the P0 foundation is on main:

• Resolve exact Git pre/post blobs and diff semantics #21 — exact immutable Git pre/post inputs
• Build deterministic Tree-sitter DiffGraph baseline with evidence #22 — deterministic Tree-sitter topology with evidence
• Make one validated, versioned DiffGraph artifact the canonical CLI contract #23 — canonical versioned, schema-validated DiffGraph artifact

Scope

• Add a small provider/stage abstraction that consumes the canonical frozen artifact.
• Keep deterministic symbols and relationships authoritative and unchanged.
• Permit optional AI/provider stages to add human-readable explanations only.
• Support AI-off/local operation as the baseline.
• Add explicit privacy tiers and one-time consent before any code/diff/artifact leaves the machine.
• Keep provider internals out of the CLI and renderer contracts.

Acceptance criteria

• Every provider accepts a schema-valid canonical artifact produced by the deterministic pipeline.
• Provider output cannot add, remove, or alter symbols, edges, evidence, or provenance.
• --no-ai/local operation remains fully functional.
• Cloud use clearly identifies what is sent and requires consent before first transmission.
• Tests prove topology is byte-for-byte unchanged with providers enabled/disabled.
• Documentation describes only implemented, verified behavior.

Supersedes

This is the direction-aligned replacement scope for #12, whose old branch combined useful extensibility/privacy work with AI-owned topology, a placeholder default mode, and pre-canonical contracts.