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/config.env

@@ -0,0 +1,6 @@
+# kat instance configuration
+FLY_APP="kat-org-agenda-api"
+GIT_SYNC_REPOSITORIES='[{"url":"ssh://gitea@dev.railbird.ai:1123/colonelpanic/katnivan.git","path":"org"},{"url":"ssh://gitea@dev.railbird.ai:1123/kkathuang/org.git","path":"shared"}]'
+GIT_USER_EMAIL="kat-org-agenda-api@colonelpanic.io"
+GIT_USER_NAME="kat-org-agenda-api"
+AUTH_USER="kat"