[codex] add state query helpers

[pdurlej]

Summary

• add state query helpers for Anytime, Someday, Upcoming, headings, heading children, tags, and text search
• switch Today range calculation to local-day boundaries instead of duration truncation
• add regression coverage for the new query helpers and QueryOpts filtering
• update README state-query examples to match the implemented API

Context

The README already advertised TasksInAnytime, but sync.State only exposed Inbox, Today, Project, and Area task queries. These helpers make the persistent sync state more usable for clients that need Things-style views, including OpenClaw-style integrations.

SearchTasks is intentionally a simple SQLite LIKE search over title and note. It is not a full-text index.

Validation

• go test ./sync -run TestStateQueries -count=1
• go test ./...