feat(rag): summarize single-document folders (MIN_DOCS_PER_FOLDER 2 → 1)

Relax the phase-94 folder-summary scope rule from ≥ 2 documents to
≥ 1: a folder (or source root) is a candidate while ANY document
lives under it, so single-file folders and single-file source roots
get their own lite-written description. A row is now pruned only
when its folder loses its last document (vanishes from the
catalogue).

The constant is the single source of truth, so the flip propagates
to the generator's candidate set, the prune pass, the
missing_folder_summaries gap probe (the next sync self-heals the
new gaps), and the KB-tree summary_pending markers (1-doc folders /
sources now read "Summary pending" until their row lands).

Docstrings/comments across app/, scripts/import_docs.py, and the
E2E fixtures updated to the ≥ 1 wording. Unit + integration tests
updated to the new semantics (the pruned-below-minimum scenario is
now a folder losing its LAST doc; single-doc folders are pinned as
candidates/pending). Full suite: 2314 passed, app coverage 99%;
ruff + pyright clean; folder-summary E2E stories pass in isolation
(ls_tree_drilldown, sync_summary_visibility, kb_tree,
kb_tree_nav, document_dates, oneshot_llm_retry).

tests/e2e/test_ls_tree_drilldown.py

@@ -42,7 +42,8 @@ of the mock's cosine distribution, and the ``<tools>`` section is present
 (the flow's precondition).
 
 The mock's canned ``FOLDER_SUMMARY_MODE`` branch (phase 94 task 01)
-stores, per ≥ 2-doc folder, the deterministic one-liner
+stores, per existing folder (the ≥ 1-doc rule — this fixture holds
+no single-doc folders), the deterministic one-liner
 ``Fixture folder summary for <source>[/<folder>].`` — the ``synced_kb``
 module fixture pins those exact rows in ``folder_summaries`` after the
 sync, and the drill answers assert on them in the ``ls`` output.
@@ -159,8 +160,9 @@ assert "\n" not in TWO_A_CONTENT[:80]  # the quote must stay one line
 
 #: The sync-time folder summaries the mock's canned ``FOLDER_SUMMARY_MODE``
 #: branch stores (task 01's byte-stable template), in
-#: ``(source, folder_path)`` order: one row per ≥ 2-doc folder (the
-#: recursive-subtree rule) — the ``""`` rows are the source roots.
+#: ``(source, folder_path)`` order: one row per existing folder (the
+#: ≥ 1-doc recursive-subtree rule — this fixture holds no single-doc
+#: folders) — the ``""`` rows are the source roots.
 EXPECTED_SUMMARIES: list[tuple[str, str, str]] = [
     (ALPHA, "", f"Fixture folder summary for {ALPHA}."),
     (ALPHA, "one", f"Fixture folder summary for {ALPHA}/one."),
@@ -426,7 +428,8 @@ def synced_kb(app_server: str, drill_dirs: tuple[Path, Path]) -> None:
     walk → chunk → embed → overview → folder summaries → version bump),
     and pins the stored folder summaries: the mock's canned
     ``FOLDER_SUMMARY_MODE`` branch (task 01) makes the sync store one
-    deterministic row per ≥ 2-doc folder — the drill turns' answers
+    deterministic row per existing folder (the ≥ 1-doc rule — this
+    fixture holds no single-doc folders) — the drill turns' answers
     assert on that exact text.
     """
     alpha, beta = drill_dirs
@@ -452,7 +455,8 @@ def synced_kb(app_server: str, drill_dirs: tuple[Path, Path]) -> None:
     assert detail["pruned"] == 0, detail
     assert detail["overview"] is True, detail
     # The change-gated folder summaries (phase 94 task 02) landed: one
-    # row per ≥ 2-doc folder, the mock's byte-stable text (the drill
+    # row per existing folder (the ≥ 1-doc rule — this fixture holds
+    # no single-doc folders), the mock's byte-stable text (the drill
     # answers quote exactly these lines).
     with SessionLocal() as db:
         rows = db.execute(