ci: add gitflow — dev/stage/main branches with CI workflows

- dev.yaml: build + local docker smoke test only (no push, no deploy)
- stage.yaml: build + push + deploy to marketing-stage + smoke test (stops here)
- deploy.yaml: add HTML placeholder touch step before docker build

Proper human gate between stage and prod: the stage→main merge decision.

runtime/el_runtime.h

@@ -739,6 +739,12 @@ el_val_t  map_set(el_val_t map, el_val_t key, el_val_t value); /* el_map_set */
 /* See bottom of el_runtime.c for the implementation.
  * Configured by env vars OTLP_ENDPOINT, OTEL_SERVICE_NAME, OTEL_SERVICE_VERSION.
  * No-op when OTLP_ENDPOINT is unset. Drop-on-failure semantics. */
+/* ── Subprocess execution ────────────────────────────────────────────────── */
+el_val_t exec_command(el_val_t cmd);   /* run shell command, return exit code */
+el_val_t exec_capture(el_val_t cmd);   /* run shell command, capture stdout   */
+el_val_t exec(el_val_t cmd);           /* exec(cmd) → stdout String (30s timeout) */
+el_val_t exec_bg(el_val_t cmd);        /* exec_bg(cmd) → PID String (non-blocking) */
+
 el_val_t emit_log(el_val_t level, el_val_t msg, el_val_t fields_json);
 el_val_t emit_metric(el_val_t name, el_val_t value, el_val_t tags_json);
 el_val_t trace_span_start(el_val_t name);