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/cmd_test.go

@@ -95,3 +95,12 @@ func TestInitFlagSetScopes(t *testing.T) {
 		})
 	}
 }
+
+func TestInitFlagSetHealthcheck(t *testing.T) {
+	t.Cleanup(func() { healthcheck = false })
+	fs := flag.NewFlagSet("test", flag.ContinueOnError)
+	initFlagSet(fs, []string{"-healthcheck"}, func(string) string { return "" }, func(string) ([]byte, error) { return nil, nil }, &bytes.Buffer{})
+	if !healthcheck {
+		t.Error("healthcheck = false, want true")
+	}
+}