fix: replace partial pattern in readProcessBytes with explicit case

Also adds type-safety conventions to AGENTS.md: no partial patterns on
IO results, no partial functions (head/tail/fromJust), always use case
with explicit branches.

AGENTS.md

@@ -86,3 +86,28 @@ mkdir -p .stack-root
 - When adding a new subproject (e.g. `roux-server`), initialize it with `./hs stack new`.
 - Use `hpack` (via `./hs hpack`) for package metadata in `package.yaml` format.
 - When agent-created files are owned by root, fix ownership with `sudo chown -R $(id -u):$(id -g) .`.
+
+### Type safety and pattern matching
+
+This project compiles with `-Wall -Werror`, including `-Wincomplete-uni-patterns`
+and `-Wincomplete-record-updates`.  Partial functions and irrefutable pattern
+matches (`let (Just x) = ...`, `(Just x, _) <- ...`) will fail at runtime or
+be rejected by the compiler.
+
+**Rules:**
+
+- **Never** use irrefutable/partial pattern matches on `Maybe`, `Either`, or
+  tuple results from `IO` actions.  Always use `case` with an explicit branch
+  for each constructor, or use pattern-matching helpers (`maybe`, `either`).
+- **Never** call partial functions: `head`, `tail`, `init`, `last`, `fromJust`,
+  `read` (use `readMaybe`), `succ`, `pred`, `(!!)`.  Use total alternatives:
+  `foldl'` for `head`/`tail` on lists, `Data.Map.lookup` for map access,
+  `take 1` or pattern matching on `NonEmpty`.
+- **Never** use `error`, `undefined`, or `panic` for recoverable failures.
+  Use `Either`, `Maybe`, or explicit `IO` error handling (`ioError`,
+  `Control.Exception.throwIO`).
+- **Always** fully handle all constructors in case expressions. Use a wildcard
+  (`_ ->`) only when you genuinely don't care about the value, and add a
+  comment explaining why.
+- **Prefer** `case` over `<-` pattern binds in `do` notation when the pattern
+  could conceivably fail at runtime.

src/Roux/Server.hs

@@ -243,13 +243,19 @@ readProcessBytes cmd args = do
                 { std_out = CreatePipe
                 , std_err = CreatePipe
                 }
-    (Just outH, Just errH, _, ph) <- createProcess process
-    hSetBinaryMode outH True
-    hSetBinaryMode errH True
-    out <- LB.hGetContents outH
-    err <- LB.hGetContents errH
-    ec <- waitForProcess ph
-    return (ec, out, err)
+    result <- createProcess process
+    case result of
+        (Just outH, Just errH, _, ph) -> do
+            hSetBinaryMode outH True
+            hSetBinaryMode errH True
+            out <- LB.hGetContents outH
+            err <- LB.hGetContents errH
+            ec <- waitForProcess ph
+            pure (ec, out, err)
+        _ ->
+            ioError $
+                userError $
+                    "[roux] createProcess did not return std_out/std_err pipes for " <> cmd
 
 -- | Locate the scrape-recipe script: check PATH first, then relative to project.
 findScrapeScript :: IO FilePath