chore: initial project scaffolding

Add project configuration, documentation and development tooling:
- flake.nix dev shell with Go, golangci-lint, turbojpeg
- .envrc sourcing sops-encrypted .secrets.env
- golangci-lint v2 config
- opencode.json MCP + LSP wiring
- AGENTS.md development guidelines

flake.nix

@@ -0,0 +1,44 @@
+{
+  description = "TeleportFling Development Environment";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          inputsFrom = [];
+          nativeBuildInputs = with pkgs; [
+            go
+            golangci-lint
+            pkg-config
+            gopls
+            nixd
+            nodejs
+            gitea-mcp-server
+          ];
+          buildInputs = with pkgs; [
+            git
+            direnv
+            nix-direnv
+            libjpeg_turbo
+            pipewire
+            xdg-desktop-portal
+          ];
+
+          # Make turbojpeg resolve via pkg-config for cgo builds.
+          PKG_CONFIG_PATH = "${pkgs.libjpeg_turbo.dev}/lib/pkgconfig";
+          LD_LIBRARY_PATH = "${pkgs.libjpeg_turbo.out}/lib";
+
+          shellHook = ''
+            echo "TeleportFling dev shell ready!"
+          '';
+        };
+      });
+}