Add org-agenda-api container builds and fly.io deployment

Consolidates container builds from colonelpanic-org-agenda-api repo:
- Add org-agenda-api input to nixos flake
- Add container-colonelpanic and container-kat package outputs
- Add org-agenda-api cachix as substituter
- Add org-agenda-api devShell for deployment work

New org-agenda-api directory contains:
- container.nix: Container build logic using mkContainer
- configs/: Instance configs (custom-config.el, fly.toml, secrets)
- deploy.sh: Fly.io deployment script
- secrets.nix: agenix secret declarations

Build with: nix build .#container-colonelpanic
Deploy with: cd org-agenda-api && ./deploy.sh colonelpanic

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

org-agenda-api/configs/kat/secrets/auth-password.age

@@ -0,0 +1,5 @@
+age-encryption.org/v1
+-> ssh-ed25519 Tp0Z1Q ZvS3ewY5ZCm7rWQeliPHPXnzSpfFeqK/1a7pWY/l83s
+gJoiP/tSEsPYrxiVFsD1eIRPALL2tdKJFWBNMj/dpAk
+--- lN9hXwr2IFAJjhe/u52xiOpGTaDU/fWXhhquOhgBc8U
+<}t<><74><EFBFBD>]<5D> <20>0@<40>y՝z?Y~R4?"<22><>޵<0F>D9<44>X<EFBFBD><58>p}C<>