Use org-agenda-api from GitHub instead of local path

- Change org-agenda-api input from path:../dotfiles/emacs.d/straight/repos/org-agenda-api to github:colonelpanic8/org-agenda-api
- Refactor org-agenda package building to separate org-agenda-api.nix file
- Update flake.lock with new input and fix hercules-ci-effects metadata mismatch

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

nixos/flake.lock

@@ -1756,11 +1756,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1768413426,
-        "narHash": "sha256-JqIQYMO3a0J7UPS7edXie8fVrSMX+mMZKJjyGvNbZeE=",
+        "lastModified": 1768420684,
+        "narHash": "sha256-0f5aQ9vkIL8gHHUntCv6KWlZXatOlU2lzW8wuDi1rXg=",
         "owner": "colonelpanic8",
         "repo": "org-agenda-api",
-        "rev": "f342ee0b73ccd48a7aff12ca07bd742a96c1776e",
+        "rev": "8b499275f31436f36ca4d775f611208edab72d51",
         "type": "github"
       },
       "original": {