2.x gap: wire compare filters and pre-query ingest parity

[willwashburn]

Current 2.x status:

• burn compare supports model list, --since, --project, --session, --min-sample, --fidelity, --include-partial, --csv, --json, and --no-archive.
• --provider, --workflow, and --agent are now accepted by clap but rejected at runtime with explicit "not yet wired" errors (see crates/relayburn-cli/src/commands/compare.rs:119-123 and :150-154).

Missing/different from 1.x parity:

• --provider is a runtime no-op: the SDK's provider filter is private to the analyze module and isn't exposed through a typed top-level surface.
• --workflow / --agent are runtime no-ops: the Rust ledger query layer doesn't expose stamp-based enrichment filtering.
• The command skips the 1.x pre-query ingest pass, and the decision isn't documented.

Acceptance:

• Wire provider/workflow/agent filters through the Rust query layer and CLI (remove the runtime rejection branches).
• Decide whether compare should keep or intentionally drop pre-query ingest, and document the decision.
• Add tests for each filter and the chosen ingest behavior.
• Update README/help to match.