Integrate into Phinder for discovery queries

[jzonthemtn]

Summary

Update Phinder to accept PhiSQL discovery queries as input. Phinder already produces structured findings; this issue adds the query layer that lets users ask declarative questions over those findings.

Scope

• Phinder accepts PhiSQL queries via CLI (`phinder query -f query.phisql`) and API
• Findings exposed as a queryable table (`phinder.findings`)
• Storage backends remain unchanged; PhiSQL executes against whatever Phinder is already using
• Aggregation and filtering executed via the reference implementation's query engine
• Documentation updated with PhiSQL examples

Acceptance Criteria

• Phinder accepts and executes PhiSQL discovery queries
• All discovery verbs from the spec are supported
• At least 5 example queries documented (find PII in a bucket, group by entity type, top-risk findings, recent scans, cross-scan diffs)
• Performance acceptable on a 10M-finding corpus
• Tests cover query parsing, execution, and result correctness