data_diff: configurable MAX_STEPS and wall-clock timeout

Flagged during v0.5.22 release review (End User + Tech Lead personas).

`MAX_STEPS = 200` in `packages/opencode/src/altimate/native/connections/data-diff.ts` is hardcoded. The error message now suggests `partition_column` / `where_clause` remediations (good), but:

- No per-query wall-clock timeout — runaway cross-database HashDiff can hang and rack up warehouse cost
- No user override for legitimate billion-row table cases
- No telemetry breadcrumb when we hit the cap

Proposed:
- `data_diff_max_steps` config, default 200
- Per-SQL-step wall-clock timeout (60s default, configurable)
- Emit telemetry event on cap hit so we know how often users need more headroom

Deferred because: no reported hits in the field yet. File for tracking.