fix: remove committed private keys and add gitleaks CI

- Remove all cert files from git tracking (git rm --cached)
  - crates/pm-agent-client/certs/client.key (private key)
  - crates/pm-agent-client/certs/client.crt (public cert)
  - crates/pm-agent-client/certs/ca.crt (public cert)
- Add .gitignore patterns for *.key, *.key.pem, certs/*.crt, certs/*.pem
- Update pm-agent-client doc examples to use std::fs::read() instead of include_bytes!
- Add gitleaks secret scanning job to CI workflow
- Update security-review.md with critical finding for Issue #12
- Add README.md to crates/pm-agent-client/certs/ explaining runtime cert generation

Private keys were dev/test only - no production key rotation needed.
Git history purge with filter-repo will follow after PR merge.

Co-authored-by: Draco Lunaris <331325+Draco-Lunaris@users.noreply.github.com>

.gitignore

@@ -28,5 +28,9 @@ frontend/dist
 *.deb
 package-build/
 
-# TLS certificates - generated on first run
-crates/pm-agent-client/certs/
+# Private key material - NEVER commit
+*.key
+*.key.pem
+crates/pm-agent-client/certs/*.crt
+crates/pm-agent-client/certs/*.key
+crates/pm-agent-client/certs/*.pem