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=[]).

CLAUDE.md

@@ -13,8 +13,11 @@ Lexikon-seed: `injection-patterns.mjs` fra `llm-security`-pluginen.
 
 Repoet er på **v0.2 (alpha)**: stdlib-kjernen er bygget og testet (12 moduler +
 topp-nivå wiring, showcase + korpus), inkl. OKF-adapter og aktivt-innhold-
-detektor (EchoLeak-klassen) i output-gaten. Start med `docs/BRIEF.md` for design,
-`README.md` for bruk, `docs/PLAN.md` for byggerekkefølgen.
+detektor (EchoLeak-klassen) i output-gaten. Mode-b `import_bundle` skanner
+reserverte strukturfiler (`index.md`/`log.md`) i mottatte bundles i stedet for å
+path-avvise dem; upload-front-end beholder shadow-reject (`allow_reserved=False`).
+Start med `docs/BRIEF.md` for design, `README.md` for bruk, `docs/PLAN.md` for
+byggerekkefølgen.
 
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