plan(SP22): Task 4 — distinguish 4xx (terminal) from 5xx (retry) + drop dead BACKOFF_S[2]

Update _get_typed in the plan to:
- raise immediately on 4xx (terminal)
- retry on 5xx and RequestError (transient)
- use BACKOFF_S = (1.0, 2.0) instead of (1.0, 2.0, 4.0)
- use RuntimeError instead of assert for the context-manager guard

Also add test_4xx_is_terminal_not_retried to the plan's test list.

docs/superpowers/plans/2026-06-21-cyclone-pipeline-agent.md

@@ -747,7 +747,7 @@ T = TypeVar("T", bound=BaseModel)
 
 DEFAULT_TIMEOUT = httpx.Timeout(connect=10.0, read=30.0, write=10.0, pool=5.0)
 MAX_RETRIES = 3
-BACKOFF_S = (1.0, 2.0, 4.0)
+BACKOFF_S = (1.0, 2.0)  # only used between attempts 0→1 and 1→2
 
 
 # --- Response models ---------------------------------------------------------
@@ -820,24 +820,31 @@ class CycloneClient:
     async def _get_typed(
         self, path: str, model: type[T]
     ) -> T:
-        assert self._http is not None, "use `async with CycloneClient(...)`"
+        if self._http is None:
+            raise RuntimeError("use `async with CycloneClient(...)`")
         last_exc: Exception | None = None
         for attempt in range(MAX_RETRIES):
             try:
                 resp = await self._http.get(path)
                 resp.raise_for_status()
                 return model.model_validate(resp.json())
-            except (httpx.HTTPStatusError, httpx.RequestError) as e:
+            except httpx.HTTPStatusError as e:
+                # 4xx is terminal — surface immediately, do not retry
+                if e.response is not None and e.response.status_code < 500:
+                    raise
                 last_exc = e
-                if attempt < MAX_RETRIES - 1:
-                    wait_s = BACKOFF_S[attempt]
-                    log.warning(
-                        "api_get_retry",
-                        path=path, attempt=attempt + 1, wait_s=wait_s,
-                        error=str(e),
-                    )
-                    await asyncio.sleep(wait_s)
-        assert last_exc is not None
+            except httpx.RequestError as e:
+                # connect/read failures are transient — retry
+                last_exc = e
+            if attempt < MAX_RETRIES - 1:
+                wait_s = BACKOFF_S[attempt]
+                log.warning(
+                    "api_get_retry",
+                    path=path, attempt=attempt + 1, wait_s=wait_s,
+                    error=str(last_exc),
+                )
+                await asyncio.sleep(wait_s)
+        assert last_exc is not None  # loop body always sets this on the way out
         raise last_exc
 
     # --- methods -----------------------------------------------------------