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

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

.github/copilot-instructions.md

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+# Copilot instructions — httpx
+
+## Project
+
+`git.codelab.vc/pkg/httpx` is a Go 1.24 HTTP client and server library with zero external dependencies.
+
+## Architecture
+
+### Client (`httpx` root package)
+- Middleware type: `func(http.RoundTripper) http.RoundTripper`
+- Chain assembly (outermost → innermost): Logging → User MW → Retry → Circuit Breaker → Balancer → Transport
+- Retry wraps CB+Balancer so each attempt can hit a different endpoint
+- Circuit breaker is per-host (sync.Map of host → Breaker)
+- Client.Close() required when using WithEndpoints() — stops health checker goroutine
+
+### Server (`server/` package)
+- Middleware type: `func(http.Handler) http.Handler`
+- Router wraps http.ServeMux with groups, prefix routing, Mount for sub-handlers
+- Defaults() preset: RequestID → Recovery → Logging + production timeouts
+- Available middleware: RequestID, Recovery, Logging, CORS, RateLimit, MaxBodySize, Timeout
+- WriteJSON/WriteError for JSON responses
+
+### Sub-packages
+- `middleware/` — client-side middleware (Logging, Recovery, Auth, Headers, RequestID)
+- `retry/` — retry with exponential backoff and Retry-After
+- `circuitbreaker/` — per-host circuit breaker
+- `balancer/` — load balancing with health checking
+- `internal/requestid/` — shared context key between server and middleware
+- `internal/clock/` — deterministic time for tests
+
+## Conventions
+
+- All configuration uses functional options (`WithXxx` functions)
+- Zero external dependencies — do not add requires to go.mod
+- Tests use stdlib only (testing, httptest) — no testify or gomock
+- Thread safety with sync.Mutex, sync.Map, or atomic
+- Client test mock: `mockTransport(fn)` using `middleware.RoundTripperFunc`
+- Server test helpers: `httptest.NewRecorder`, `httptest.NewRequest`
+- Do NOT import server from middleware or vice versa — use internal/requestid for shared context
+- Sentinel errors in sub-packages, re-exported in root package
+- Use internal/clock for time-dependent tests
+
+## Commands
+
+```bash
+go build ./...           # compile
+go test ./...            # test
+go test -race ./...      # test with race detector
+go vet ./...             # static analysis
+```