Open-Connector integration proposal (#151)

* Document Open Connector integration design

Define the relational adapter, SQL interfaces, safety model, source-pack rollout, and milestone acceptance criteria so implementation can proceed in independently reviewable phases.

* docs(open-connector): drop SQL DDL registration, address review feedback

Address PR #151 review comments:

- Replace CREATE EXTERNAL TABLE registration with context-YAML-only
  binding via a DataFusion CatalogProvider. The SQL validator's no-DDL
  invariant stays intact and the shared server SessionContext gets an
  operator-controlled catalog instead of ad-hoc per-user DDL. DDL moves
  to Alternatives Considered and Future Extensions.
- Cache only scans that run to natural pagination exhaustion so
  truncated LIMIT scans can never be served as complete results.
- Enforce read-only classification from Open Connector action metadata
  with default-deny, independent of the user-editable allowlist.
- Surface source-pack versions at registration time; add an
  illustrative github.issues source-pack definition showing where the
  schema, pagination, and filter_pushdown contracts live.
- Document pack registry resolution and a future user-authored
  source-pack tier loaded like pipelines and jobs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: BtXin