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_kb_tree.py

@@ -26,7 +26,9 @@ real-Sync pattern; no git anywhere):
 Total: 7 documents; alpha counts 5 (root + 2 + 2), beta counts 2. Every
 stored description is deterministic: the mock's EXISTING
 ``FOLDER_SUMMARY_MODE`` branch (phase 94 — no mock changes needed)
-stores, per ≥ 2-doc folder, the canned one-liner naming the folder,
+stores, per existing folder (the ≥ 1 rule — this fixture holds no
+single-doc folders, so the pinned rows are unchanged), the canned
+one-liner naming the folder,
 ``Fixture folder summary for <source>[/<folder>].`` — the ``synced_kb``
 module fixture pins those exact rows (all with
 ``manually_edited = false``) after the sync, and the tests assert on
@@ -154,8 +156,9 @@ TOTAL_DOCS = ALPHA_COUNT + BETA_COUNT
 #: The sync-time folder descriptions the mock's canned
 #: ``FOLDER_SUMMARY_MODE`` branch stores (the phase-94 byte-stable
 #: template — the one-liner names the folder), in ``(source,
-#: folder_path)`` order: one row per ≥ 2-doc folder (the
-#: recursive-subtree rule) — the ``""`` rows are the source roots.
+#: 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.
 SUMMARY_FOR = "Fixture folder summary for {}."
 ALPHA_ROOT_SUM = SUMMARY_FOR.format(ALPHA)
 ONE_SUM = SUMMARY_FOR.format(f"{ALPHA}/one")
@@ -382,7 +385,8 @@ def synced_kb(app_server: str, kb_tree_dirs: tuple[Path, Path]) -> None:
     walk → chunk → embed → overview → folder summaries → version
     bump), and pins the stored folder descriptions: the mock's canned
     ``FOLDER_SUMMARY_MODE`` branch (phase 94) makes the sync store one
-    deterministic row per ≥ 2-doc folder — the tests assert on that
+    deterministic row per existing folder (the ≥ 1-doc rule — this
+    fixture holds no single-doc folders) — the tests assert on that
     exact text (and on the phase-97 ``manually_edited`` flag: every
     stored row starts out AI-written).
     """
@@ -409,7 +413,8 @@ def synced_kb(app_server: str, kb_tree_dirs: tuple[Path, Path]) -> None:
     assert detail["pruned"] == 0, detail
     assert detail["overview"] is True, detail
     # The change-gated folder descriptions (phase 94) landed: one row
-    # per ≥ 2-doc folder, the mock's byte-stable text — and every row
+    # per existing folder (the ≥ 1-doc rule — this fixture holds no
+    # single-doc folders), the mock's byte-stable text — and every row
     # AI-written (the ``manually_edited`` flag starts false).
     with SessionLocal() as db:
         rows = db.execute(