Ship the runnable SQL Server example

[victorchutw]

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

Blocked by #136

Context

Every example is executed against its README's claims by tests/examples.rs. The SQL Server example keeps that invariant wherever a server exists — in CI, always — by carrying its executing test behind the same ignore gate. Full refresh is the example's mode: it is the one mode that bootstraps its own table, so the example needs no pre-existing DDL.

The contract

• examples/csv-to-sqlserver-full-refresh/: a small CSV fixture + a version: 1 load definition using the sqlserver destination (password_env posture; host/port/user matching the documented container defaults; trust_server_certificate: true for the self-signed container cert, stated plainly in the README) + a README saying what it demonstrates.
• The README states the server prerequisite and the docker one-liner verbatim from CONTRIBUTING.md (Add a minimal CONTRIBUTING.md as the contributor home #133) — the examples' zero-prerequisite framing relaxes for server-backed destinations only, and says so.
• tests/examples.rs: a SQL Server destination arm reading the table back through tiberius (row count + a value probe), and the example's executing entry behind #[ignore = "needs SQL Server"], connecting with the same fixed defaults / DATA_SPARK_TEST_MSSQL_* overrides as the connector's own gated tests.
• examples/README.md: a table row for the new example, plus the one framing adjustment: self-contained phrasing now notes that server-backed examples state their prerequisite.

Acceptance criteria

1. The example runs as its README says on a machine with the container up: one command sequence — start container, export the password env var, run the load — exits 0 with the documented report facts (atomic / transactional_delete_insert).
2. The gated examples entry executes in CI (via --include-ignored) and reads the rows back through tiberius; on a serverless machine it shows up as ignored and plain cargo test --locked stays green.
3. examples/README.md lists the example and carries the adjusted framing sentence.
4. CHANGELOG.md Unreleased entry (examples are user-visible per the changelog's scope note).