Add a SQL Server read-back posture to the verify skill

[victorchutw]

Implements ADR-0066 (the verify-skill surface). Part of the SQL Server destination cycle (map #106, sliced from #113).

Blocked by #136

Context

The verify skill (.claude/skills/verify/SKILL.md) reads destinations back independently of the CLI — Parquet and DuckDB via Python DuckDB today. SQL Server needs a read-back posture with zero new host dependencies.

The contract

SKILL.md gains a SQL Server posture:

• Read the destination back through the running test container: docker exec <container> /opt/mssql-tools18/bin/sqlcmd -C -U sa -P <the documented test password> -d <database> -Q "SELECT ..." — no new host tools, matching the documented container one-liner from CONTRIBUTING.md (Add a minimal CONTRIBUTING.md as the contributor home #133).
• Extend the report-versus-destination comparison step to the sqlserver write postures (atomic/transactional_delete_insert, best_effort/bulk_insert, atomic/transactional_merge).
• Extend the failure-boundary probe list with one sqlserver case: an incompatible existing table (incompatible_destination_table) leaving destination rows untouched.

Keep the skill's shape and brevity — it is a numbered procedure, not a guide.

Acceptance criteria

1. The updated instructions are executed once end-to-end against the documented container (evidence in the PR: commands + output) and read back a real load's rows via docker exec … sqlcmd.
2. The skill stays accurate for the existing DuckDB/Parquet postures — no regressions in the procedure.
3. No CHANGELOG entry (agent tooling is not a user-facing surface).