fix(chat): stop ArrowUp from eating an unsent multi-line prompt (#5875)

static/app.js carried a near-verbatim copy of the prompt-recall logic in
static/js/composerArrowUpRecall.js, wired as a second capture-phase
keydown listener on the same #message textarea. The copy omitted the
draft guard the module has: it called preventDefault() and
stopImmediatePropagation() unconditionally, then recalled history[0]
over whatever the user had typed.

Because it stopped immediate propagation, the copy won regardless of
registration order — if it ran first the module never saw the event, and
if it ran second the module had already declined to stop propagation on
an unmatched draft. The guard at composerArrowUpRecall.js:109 was
unreachable on the real page, so ArrowUp on a multi-line draft replaced
it with the last sent prompt instead of moving the caret up a line.

Delete the duplicate. The module keeps ownership of ArrowUp/ArrowDown
recall, which is the behavior MODULE_SUMMARY.md documents ("on an empty
composer") and the behavior tests/test_composer_arrow_up_recall_js.py
already pins via test_non_empty_composer_does_not_recall and
test_multiline_caret_navigation_preserved.

Also correct a stale comment in the module that described the deleted
behavior and contradicted the guard 35 lines above it, and add a
regression test asserting app.js does not reintroduce a second handler.

Fixes #5862

tests/test_composer_arrow_up_recall_js.py

@@ -306,3 +306,24 @@ def test_integration_recalls_from_chat_history_dom():
     )
     assert proc.returncode == 0, proc.stderr
     assert json.loads(proc.stdout.strip()) == {"value": "stored prompt", "prevented": True}
+
+
+def test_prompt_recall_is_not_duplicated_in_app_js():
+    """Only composerArrowUpRecall.js may own ArrowUp on #message (issue #5862).
+
+    static/app.js once carried a near-verbatim copy of this recall logic, wired
+    as a second capture-phase listener on the same textarea. That copy lacked
+    the draft guard here, and because it called stopImmediatePropagation it won
+    regardless of registration order — so a typed multi-line prompt was replaced
+    by the last sent one instead of the caret moving up a line.
+    """
+    app_js = (_REPO / "static" / "app.js").read_text(encoding="utf-8")
+    for marker in (
+        "_odysseusPromptRecallCapture",
+        "_readComposerPromptHistory",
+        "odysseusRecallIndex",
+    ):
+        assert marker not in app_js, (
+            f"static/app.js reintroduces prompt recall ({marker!r}); "
+            "it belongs to static/js/composerArrowUpRecall.js alone"
+        )