Add check-security-patterns.sh + CI job that hard-fails if any PR
introduces new shell execution (Command::new("sh"/"bash"/"cmd")) or
unsafe {} blocks in Rust source.
Scans only added lines via `git diff --unified=0 --diff-filter=AM` +
`grep '^+'` — never flags pre-existing code (e.g. runner.rs issue #640).
Also adds `-D unsafe_code` to the clippy job: RTK currently has zero
unsafe blocks, so this is free to enforce now.
Warn-only (non-blocking) for new .unwrap() and println! in filter code.
--self-test mode available for local verification.
Closes no issue — terrain vierge (verified: no prior PR/issue for this).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Florian BRUNIAUX <florian@bruniaux.com>