Add AI agent configuration files (AGENTS.md, .cursorrules, copilot-instructions)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.cursorrules

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+You are working on `git.codelab.vc/pkg/httpx`, a Go 1.24 HTTP client/server library with zero external dependencies.
+
+## Architecture
+
+- Client middleware: `func(http.RoundTripper) http.RoundTripper` — compose with `middleware.Chain`
+- Server middleware: `func(http.Handler) http.Handler` — compose with `server.Chain`
+- All configuration uses functional options pattern (`WithXxx` functions)
+- Chain order for client: Logging → User MW → Retry → Circuit Breaker → Balancer → Transport
+
+## Package structure
+
+- `httpx` (root) — Client, request builders (NewJSONRequest, NewFormRequest), error types
+- `middleware/` — client-side middleware (Logging, Recovery, Auth, Headers, RequestID)
+- `retry/` — retry middleware with exponential backoff and Retry-After support
+- `circuitbreaker/` — per-host circuit breaker (sync.Map of host → Breaker)
+- `balancer/` — load balancing with health checking (RoundRobin, Weighted, Failover)
+- `server/` — Server, Router, server middleware (RequestID, Recovery, Logging, CORS, RateLimit, MaxBodySize, Timeout), response helpers (WriteJSON, WriteError)
+- `internal/requestid/` — shared context key (avoids circular import between server and middleware)
+- `internal/clock/` — deterministic time for tests
+
+## Code conventions
+
+- Zero external dependencies — stdlib only, do not add imports outside the module
+- Functional options: `type Option func(*options)` with `With<Name>` constructors
+- Test with stdlib only: `testing`, `httptest`, `net/http`. No testify/gomock
+- Client test helper: `mockTransport(fn)` wrapping `middleware.RoundTripperFunc`
+- Server test helper: `httptest.NewRecorder`, `httptest.NewRequest`, `waitForAddr(t, srv)`
+- Thread safety with `sync.Mutex`, `sync.Map`, or `atomic`
+- Use `internal/clock` for time-dependent tests, not `time.Now()` directly
+- Sentinel errors in sub-packages, re-exported as aliases in root package
+
+## When writing new code
+
+- Client middleware → file in `middleware/`, return `middleware.Middleware`
+- Server middleware → file in `server/middleware_<name>.go`, return `server.Middleware`
+- New option → add field to options struct, create `With<Name>` func, apply in constructor
+- Do NOT import `server` from `middleware` or vice versa (use `internal/requestid` for shared context)
+- Client.Close() must be called when using WithEndpoints() (stops health checker goroutine)
+- Request bodies must have GetBody set for retry — use NewJSONRequest/NewFormRequest
+
+## Commands
+
+```bash
+go build ./...           # compile
+go test ./...            # test
+go test -race ./...      # test with race detector
+go vet ./...             # static analysis
+```