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opposite and is superseded in place, with the reason written down.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_inbox_flow.py

@@ -421,14 +421,21 @@ def test_an_empty_inbox_writes_nothing(tmp_path: Path) -> None:
     assert not bundle.exists() or list(bundle.iterdir()) == []
 
 
-def test_subdirectories_are_not_walked(tmp_path: Path) -> None:
-    """Top-level only, like every other inbox in this ecosystem: a nested tree
-    is the operator's structure, not ours to flatten into one namespace.
+def test_subdirectories_are_walked(tmp_path: Path) -> None:
+    """Superseded, deliberately, by the operator's decision of 2026-09-06.
+
+    This test used to assert the opposite -- top-level only, "the operator's
+    nested structure is theirs". The rule it pinned was not a boundary, it was
+    a silent loss: a file under a folder appeared in NONE of the result's
+    buckets, so the count said nothing was there rather than that nothing was
+    looked at. The nested structure is still the operator's, and it survives
+    in the recorded `source_file`; it is the disappearance that is gone. The
+    rule's own suite is `tests/test_inbox_recursion.py`.
     """
     drop(tmp_path / "inbox" / "nested", "deep.md", "Body\n")
     drop(tmp_path / "inbox", "top.md", "Body\n")
 
     result, bundle = run(tmp_path, StubGuard())
 
-    assert [entry.source_file for entry in result.persisted] == ["top.md"]
-    assert not (bundle / "inbox-deep.md").exists()
+    assert [entry.source_file for entry in result.persisted] == ["nested/deep.md", "top.md"]
+    assert (bundle / "inbox-deep.md").is_file()