Implement comprehensive CI/CD pipeline with best practices

Major improvements:
- Matrix testing across Linux (stable/beta/nightly), Windows, and macOS
- Test with multiple feature combinations (default, yara-scanning, no-default)
- Code coverage reporting with codecov integration
- Security auditing with cargo-audit and dependency review
- Automated release builds for multiple targets
- Performance benchmarking with trend tracking
- Concurrency control to cancel outdated runs
- Rust cache optimization for faster builds
- Documentation generation checks
- Weekly scheduled runs for proactive monitoring

Additional workflows:
- Automated dependency updates via Dependabot
- Weekly Cargo dependency update PRs
- Stale issue and PR management

Project templates:
- Pull request template with checklist
- Bug report issue template
- Feature request issue template
- Codecov configuration with 70% coverage target

codecov.yml

@@ -0,0 +1,27 @@
+coverage:
+  precision: 2
+  round: down
+  range: 70..100
+  status:
+    project:
+      default:
+        target: 70%
+        threshold: 5%
+        if_ci_failed: error
+    patch:
+      default:
+        target: 80%
+        threshold: 10%
+
+comment:
+  layout: "header, diff, flags, components, files, footer"
+  behavior: default
+  require_changes: false
+  require_base: false
+  require_head: true
+
+ignore:
+  - "tests/**"
+  - "benches/**"
+  - "examples/**"
+  - "**/*_test.rs"