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_sync_summary_visibility.py

@@ -46,7 +46,9 @@ the same host) registered as a ``kind=local`` source (the
 pattern; no git anywhere): ``syncsum/`` with TWO ≥ 2-doc folders —
 ``alpha/`` (2 docs) and ``bravo/`` (2 docs), no root-level files. The
 generator's candidate set is therefore exactly 3: the source root
-(``folder_path ""``) + the two folders (the recursive-subtree ≥ 2 rule).
+(``folder_path ""``) + the two folders (the recursive-subtree ≥ 1
+rule — this fixture holds no single-doc folders, so the rule's
+relaxation is inert for it).
 
 Autouse cleanup (the phase-96 pattern): before each test wait for no
 running sync on EITHER app, then truncate the shared registry + KB
@@ -160,7 +162,7 @@ FOLDER_B = "bravo"
 FOLDERS = (FOLDER_A, FOLDER_B)
 
 N_FILES = 4  # two folders × 2 docs
-N_CANDIDATES = 3  # the source root + the two folders (the ≥ 2-doc rule)
+N_CANDIDATES = 3  # the source root + the two folders (the ≥ 1-doc rule)
 
 #: The mock's byte-stable ``FOLDER_SUMMARY_MODE`` lines for this fixture
 #: (the phase-94 template — the label is the ``FOLDER_HEADER_PREFIX``