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

.golangci.yml

@@ -0,0 +1,49 @@
+version: "2"
+
+run:
+  timeout: 5m
+  tests: true
+
+linters:
+  default: none
+  enable:
+    - errcheck
+    - govet
+    - staticcheck
+    - ineffassign
+    - unused
+    - misspell
+    - revive
+    - gosec
+    - nakedret
+    - unconvert
+    - gocritic
+  exclusions:
+    # Cgo-heavy capture/protocol code may need exceptions for specific patterns.
+    rules:
+      - path: _test\.go
+        linters:
+          - gosec
+      # Wire-format int conversions: the Teleport protocol defines 32-bit fields
+      # on the wire (uint32) while the reference uses int32 struct fields. These
+      # casts are intentional and mirror obs-teleport's types.go.
+      - path: "internal/protocol/"
+        linters:
+          - gosec
+        text: "G115"
+      # gocritic mis-parses cgo calls with multiple pointer args (&jpegSize,
+      # &dstPtr) as suspicious == comparisons.
+      - path: "internal/protocol/jpeg\\.go"
+        linters:
+          - gocritic
+        text: "dupSubExpr"
+      # Dialogue box pixel math is clamped by n<=4 in the avg loop.
+      - path: "cmd/teleportfling/"
+        linters:
+          - gosec
+        text: "G115"
+
+formatters:
+  enable:
+    - gofmt
+    - goimports