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/fly.toml

@@ -0,0 +1,32 @@
+# fly.toml - Fly.io deployment configuration
+# See https://fly.io/docs/reference/configuration/
+
+app = "kat-org-agenda-api"
+primary_region = "ord"
+
+# Image is built from flake and passed via deploy.sh --image flag
+
+[http_service]
+  internal_port = 80
+  force_https = true
+  auto_stop_machines = false
+  auto_start_machines = true
+  min_machines_running = 1
+
+  # Give services time to start (nginx + emacs)
+  [http_service.concurrency]
+    type = "connections"
+    hard_limit = 25
+    soft_limit = 20
+
+[[http_service.checks]]
+  grace_period = "10s"
+  interval = "30s"
+  timeout = "5s"
+  method = "GET"
+  path = "/health"
+
+[[vm]]
+  cpu_kind = "shared"
+  cpus = 1
+  memory_mb = 512