fix(context): reserve continuation-hint bytes within the retrieval page ceiling

A full non-final retrieval page consumed the entire byte budget and then
appended the pagination hint, so the returned result (which bypasses the
result-bounding wrapper) exceeded the ceiling. Reserve the hint's bytes out
of the page budget so content plus hint always fits.

tests/test_output_store.py

@@ -6,6 +6,7 @@ import re
 from typing import TYPE_CHECKING
 
 from strix.tools.output_store import (
+    _PAGE_MAX_BYTES,
     bound_and_store,
     bound_text,
     configure_output_store,
@@ -125,6 +126,21 @@ def test_read_stored_output_paginates(tmp_path: Path) -> None:
     assert "offset=10" in page
 
 
+def test_read_stored_output_page_including_hint_stays_within_ceiling(tmp_path: Path) -> None:
+    # A full non-final page plus its continuation hint must not exceed the page
+    # ceiling — retrieval bypasses the general result-bounding wrapper.
+    configure_output_store(tmp_path)
+    text = "x" * (_PAGE_MAX_BYTES * 3)
+    output_id = re.search(
+        r'output_id="([0-9a-f]{32})"',
+        bound_and_store(text, max_lines=4, max_bytes=1_000),
+    )
+    assert output_id is not None
+    page = read_stored_output(output_id.group(1), offset=0, limit=_PAGE_MAX_BYTES)
+    assert "more;" in page  # a hint was appended (non-final page)
+    assert len(page.encode("utf-8")) <= _PAGE_MAX_BYTES
+
+
 def test_read_stored_output_pages_long_lines_losslessly(tmp_path: Path) -> None:
     # A single line far larger than the page budget must be split across pages
     # (never returned whole), and paging forward must reconstruct the output