[proxy] add rtk sqlcmd (SQL Server CLI) — analogous to rtk psql

Use case

rtk psql exists and compresses Postgres table output. The Microsoft sqlcmd CLI (MSSQL) produces structurally identical noise — column header separators, padding, trailing whitespace — and is heavily used in .NET / Windows-server stacks.

Frequency

rtk discover on a 30-day Claude Code history: 208 invocations of sqlcmd, currently completely unfiltered.

Suggested behaviour (mirroring rtk psql)

• Strip ASCII table borders / dash separator rows.
• Compress repeated whitespace inside cells.
• Honour -Q/-i flags transparently.
• Optionally: prefer JSON via sqlcmd -y 0 + a JSON-aware variant, similar to what rtk aws does with --output json.

Filename suggestion: src/proxies/sqlcmd.rs, registered analogously to psql.rs.