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

.sops.yaml

@@ -0,0 +1,8 @@
+# sops creation rules for TeleportFling.
+# Encrypts .secrets.env with this machine's age key (same key used in Nix-Vibe).
+# The age private key lives at ~/.config/sops/age/keys.txt — sops finds it
+# automatically, so no SOPS_AGE_KEY_FILE override is needed here.
+creation_rules:
+  - path_regex: \.secrets\.env$
+    age: >-
+      age17z3fuzlfmerpnsrum9g4sfmkmgghtlltfq07lp79uzugm5are3cs64dnqy