feat(docker): add /healthz endpoint and -healthcheck flag (#254)

Adds a Docker healthcheck and the small HTTP/CLI plumbing it needs.

- Expose `GET /healthz` on the HTTP-mode server.
- Add `-healthcheck` to dial `http://127.0.0.1:<port>/healthz` and exit with status.
- Add a Dockerfile `HEALTHCHECK` instruction using `-healthcheck`.
- Document HTTP-mode usage and stdio override guidance.

Closes https://gitea.com/gitea/gitea-mcp/issues/146

Assisted by Codet.

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/254
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

cmd/healthcheck.go

@@ -0,0 +1,29 @@
+package cmd
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+)
+
+// runHealthcheck dials the /healthz endpoint on 127.0.0.1:port and reports
+// success or failure. It returns true when the server responds with a 2xx
+// status.
+func runHealthcheck(client *http.Client, port int, stdout, stderr io.Writer) bool {
+	url := fmt.Sprintf("http://127.0.0.1:%d/healthz", port)
+
+	resp, err := client.Get(url)
+	if err != nil {
+		fmt.Fprintf(stderr, "healthcheck failed: %v\n", err)
+		return false
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode < http.StatusOK || resp.StatusCode >= http.StatusMultipleChoices {
+		fmt.Fprintf(stderr, "healthcheck failed: unexpected status %s\n", resp.Status)
+		return false
+	}
+
+	fmt.Fprintln(stdout, "healthy")
+	return true
+}