feat(inbox): walk the drop directory recursively

Door B listed `inbox.iterdir()` and kept only top-level files. A file in a
subdirectory was neither ingested nor refused: it appeared in none of the
result's buckets, so a nested drop produced a bundle that was silently short
of what was dropped and no count said so. That broke the K1b identity for any
inbox with folders in it. Operator decision 2026-09-06.

- `walk_inbox` is the ONE walk rule, shared with `tools/okf_corpus_run.py`:
  the denominator N is now counted over exactly the set of files the door
  ingests, rather than over a second listing that happened to agree.
- Sorted on the whole relative path, not the basename, so the order is a
  function of the tree; that is what keeps rebuild-from-scratch byte-equal to
  an incremental update.
- A concept's `source_file` is the path relative to the inbox root,
  `/`-separated. The concept NAME still comes from the basename, so two
  folders holding one basename hit the existing §3 collision refusal instead
  of one silently claiming the other's concept.
- Dot-directories and a bundle directory inside the inbox are skipped with a
  CODE, in a new `InboxResult.skipped`. Recursion makes the door's own output
  reachable as its own input; a silent skip would be the same
  absence-without-a-denominator defect one level down.
- `--path-prefix` reduces per component and rejoins with `/`, so the caller
  driving a nested corpus can carry the relative directory. Reducing the whole
  string folded the separator into a `-` and flattened `sub/sub2`.

`tests/test_inbox_flow.py::test_subdirectories_are_not_walked` asserted the
opposite and is superseded in place, with the reason written down.

Measured on the K2 corpus (flat, N=43): 39/43 merged, 4 coded, K1b holds. The
bundle digest is
`1472e98aec8643c5beee540f4c42b5e437bd26e7c61d69a91bcff799f06a6d13` over 1108
files -- byte-identical to a run of the same corpus at 190086f WITHOUT this
change (`diff -r` exit 0), so recursion costs a flat inbox nothing. It differs
from the stored 2026-09-03 artifact by one line in `index.md`
(`- [Corpus run history](log.md)`), which 95eb271 added 15 hours after that
bundle was built.

Suite 1113 passed, `ruff` clean, `mypy --strict src/ tools/` clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_docs_promises.py

@@ -82,3 +82,30 @@ def test_the_readme_still_states_what_stays_out() -> None:
     text = README.read_text(encoding="utf-8")
     assert ".doc`" in text or "Word 97" in text
     assert ".doc" not in set(_PANDOC_FORMATS)
+
+
+def test_the_readme_recursion_claim_matches_the_door() -> None:
+    """The README says the drop directory is walked recursively. A sentence is
+    not a mechanism, so both halves are asserted here: the claim is in the
+    prose, and the door actually does it. Either one alone can go stale --
+    prose that outlived the code is the failure this whole module exists for.
+    """
+    import tempfile
+
+    text = README.read_text(encoding="utf-8")
+    assert "walked **recursively**" in text
+
+    from llm_ingestion_okf.inbox import GateDecision, process_inbox
+
+    with tempfile.TemporaryDirectory() as workspace:
+        inbox = Path(workspace) / "inbox" / "sub"
+        inbox.mkdir(parents=True)
+        (inbox / "deep.md").write_text("Body\n", encoding="utf-8")
+        result = process_inbox(
+            Path(workspace) / "inbox",
+            Path(workspace) / "bundle",
+            "2026-09-07T08:00:00Z",
+            okf_type="reference",
+            gate=lambda body: GateDecision(sanitized_text=body, disposition="warn", reasons=()),
+        )
+    assert [item.source_file for item in result.persisted] == ["sub/deep.md"]