Exit quietly when the reader closes stdout

[openai0229]

What

sqlx sql execute … | head -c 100 panicked with thread 'main' panicked at …/stdio.rs:1165: failed printing to stdout: Broken pipe on stderr and exit code 101, because the result and error paths print with println!, which panics when the write fails.

Both paths now go through one helper: BrokenPipe exits silently with status 0 (the consumer stopped reading, which is normal for head), any other write failure still reports itself on stderr and exits 1. The streamed event output already used fallible writes.

Verification

Large Oracle result (5000 rows), stdout piped to head -c 100:

published 0.1.6:  1 panic line on stderr
this branch:      0 lines on stderr, no panic

• Complete output is unaffected: SELECT COUNT(*) … level <= 5000 returns [['5000']].
• Successful commands and the existing tests are unchanged; cargo fmt, clippy --workspace --all-targets --locked -- -D warnings and cargo test -p ottermind-sqlx pass.

Risks and limits

• A closed pipe now exits 0; scripts that rely on a non-zero status from a truncated read must check output completeness themselves.
• Only stdout is handled; stderr write failures keep the default behaviour.