feat(httpclient): initial implementation — HTTP client with retry and circuit breaker (v1.0.0)

Introduces code.nochebuena.dev/einherjar/httpclient — the outbound HTTP client
starter for the Einherjar framework. Absorbs the httpclient package from micro-lib,
replacing fmt.Errorf wrapping with core/xerrors and adding generic JSON helpers.

Interfaces:
- Provider — Do(req *http.Request) (*http.Response, error)

Implementation:
- New(logger, cfg) Provider — configures net.Dialer + retry + circuit breaker
- NewWithDefaults(logger) Provider — convenience constructor with default config
- Retry: avast/retry-go; configurable MaxRetries and RetryDelay; retries on
  network errors and 5xx responses; logs each retry attempt at Warn level
- Circuit breaker: sony/gobreaker; opens after CBThreshold consecutive failures
  within CBTimeout window; returns ErrUnavailable when open
- DoJSON[T](ctx, client, req) (*T, error) — executes request, decodes JSON body
- DoJSONRequest[Req, Resp](ctx, client, method, rawURL, body) (*Resp, error) —
  marshals body, builds request, executes, decodes response
- MapStatusToError(code, msg) error — maps HTTP status codes to xerrors values

Config (EINHERJAR_HTTP_* env vars):
  Name(http), Timeout(30s), DialTimeout(5s),
  MaxRetries(3), RetryDelay(1s), CBThreshold(10), CBTimeout(1m)

- identifiable.go: package-level Module variable (observability.Identifiable) for version
  identification — httpclient is a stateless provider; not registered with the launcher

config.go

@@ -0,0 +1,28 @@
+package httpclient
+
+import "time"
+
+// Config holds configuration for the HTTP client.
+type Config struct {
+	// Name identifies this client in logs and circuit breaker metrics.
+	Name        string        `env:"EINHERJAR_HTTP_CLIENT_NAME"  envDefault:"http"`
+	Timeout     time.Duration `env:"EINHERJAR_HTTP_TIMEOUT"      envDefault:"30s"`
+	DialTimeout time.Duration `env:"EINHERJAR_HTTP_DIAL_TIMEOUT" envDefault:"5s"`
+	MaxRetries  uint          `env:"EINHERJAR_HTTP_MAX_RETRIES"  envDefault:"3"`
+	RetryDelay  time.Duration `env:"EINHERJAR_HTTP_RETRY_DELAY"  envDefault:"1s"`
+	CBThreshold uint32        `env:"EINHERJAR_HTTP_CB_THRESHOLD" envDefault:"10"`
+	CBTimeout   time.Duration `env:"EINHERJAR_HTTP_CB_TIMEOUT"   envDefault:"1m"`
+}
+
+// DefaultConfig returns a Config with sensible production defaults.
+func DefaultConfig() Config {
+	return Config{
+		Name:        "http",
+		Timeout:     30 * time.Second,
+		DialTimeout: 5 * time.Second,
+		MaxRetries:  3,
+		RetryDelay:  1 * time.Second,
+		CBThreshold: 10,
+		CBTimeout:   1 * time.Minute,
+	}
+}