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/container.nix

@@ -0,0 +1,43 @@
+# Container build logic for org-agenda-api instances
+# Imported by the main dotfiles flake to expose container outputs
+{ pkgs, system, tangledConfig, org-agenda-api, orgApiRev, dotfilesRev }:
+
+let
+  # Build container for a specific instance
+  mkInstanceContainer = instanceName: customConfigEl: overridesEl:
+    let
+      # Combine tangled config with instance-specific loader and overrides
+      orgAgendaCustomConfig = pkgs.runCommand "org-agenda-custom-config-${instanceName}" {} ''
+        mkdir -p $out
+
+        # Copy tangled files from dotfiles
+        cp ${tangledConfig}/*.el $out/ 2>/dev/null || true
+
+        # Add instance-specific custom-config.el loader
+        cp ${customConfigEl} $out/custom-config.el
+
+        # Add optional overrides.el if provided
+        ${if overridesEl != null then "cp ${overridesEl} $out/overrides.el" else ""}
+      '';
+    in
+    org-agenda-api.lib.${system}.mkContainer {
+      customElispFile = "${orgAgendaCustomConfig}/custom-config.el";
+      # Use content-based tag to avoid caching issues
+      tag = "${instanceName}-${orgApiRev}-${dotfilesRev}";
+    };
+
+  configsDir = ./configs;
+in
+{
+  inherit mkInstanceContainer;
+
+  # Pre-built instance containers
+  containers = {
+    colonelpanic = mkInstanceContainer "colonelpanic"
+      "${configsDir}/colonelpanic/custom-config.el"
+      null;
+    kat = mkInstanceContainer "kat"
+      "${configsDir}/kat/custom-config.el"
+      null;
+  };
+}