Clean up documentation and remove unnecessary files

- Rewrote CONTRIBUTING.md in casual, direct style
- Rewrote SECURITY.md to be less formal
- Fixed GitHub URLs in CHANGELOG.md
- Removed formal issue/PR templates
- Removed codecov.yml configuration file

All docs now sound human, not AI-generated.

.claude/settings.local.json

@@ -24,7 +24,8 @@
       "Bash(rustup override unset:*)",
       "Bash(Select-String -Pattern \"Checking|Finished|error\")",
       "Bash(Select-Object -First 20)",
-      "Bash(git restore:*)"
+      "Bash(git restore:*)",
+      "Bash(test:*)"
     ],
     "deny": [],
     "ask": []