feat(okf): scan reserved index.md/log.md in mode-b import, not path-reject (review MAJOR #2)

A received OKF bundle MAY legitimately carry index.md (directory listing, read
first under progressive disclosure) and log.md (update history) at any level
(spec §3.1/§6/§7). import_bundle previously hard-rejected those basenames in the
T4 path gate, so a conformant third-party bundle was over-blocked in full
(FAIL_SECURE) — and because the reject fired before scan_concept, index.md's
body (the highest-priority injection surface) was never scanned.

import_bundle now defaults allow_reserved=True: reserved basenames are scanned
as structural files (path-safety checks — traversal / absolute / backslash / .md
— still apply). The shadow-reject (an *upload* masquerading as index.md) is
preserved: the front-end passes allow_reserved=False so a materialized upload
landing on a reserved basename is still refused. That front-end opt-in was
required to keep the shadow-reject once the default flipped (not in the plan's
Filer set; traced from the code).

- okf.py: validate_concept_path/_validate_concept/import_bundle gain the
  keyword; validate_concept_path default stays False (strict standalone).
- tests: +3 (legit index/log admit; injection in index.md body caught;
  okf_version frontmatter admits). Per-concept-iteration test switched to a
  traversal vector; mode-b showcase's index.md surface reframed from
  reserved-name-reject to index.md-body-scan.
- README honest-limits + CLAUDE.md context note the mode-b/upload distinction.

Suite: 341 -> 344 passed. Core invariant intact (dependencies=[]).

tests/inbox_frontend.py

@@ -351,6 +351,12 @@ def receive(
     REJECT regardless of the guard's aggregate — the guard never saw that drop.
     """
     extracted = extract_inbox(paths, max_entry_bytes=max_entry_bytes, max_total_bytes=max_total_bytes)
-    result = import_bundle(extracted.bundle, origin=Origin.EXTERNAL, channel=Channel.AUTOMATIC)
+    # allow_reserved=False: these are individually-materialized *uploads*, so an
+    # upload landing on the reserved index.md/log.md is a shadow of the directory
+    # listing and is refused (T4). A received third-party bundle, by contrast,
+    # carries those as legitimate structural files (import_bundle's default).
+    result = import_bundle(
+        extracted.bundle, origin=Origin.EXTERNAL, channel=Channel.AUTOMATIC, allow_reserved=False
+    )
     verdict = "REJECT" if extracted.rejected else _VERDICT[result.disposition]
     return extracted, result, verdict