refactor(media): collapse attachment byte classification into media-core (#122168)

Attachment MIME gating and extraction dispatch used nine separate byte policies across five files; the allowlist could approve one MIME while dispatch observed another after a second sniff. @openclaw/media-core/attachment-classify now owns byte sniffing, text heuristics, extension mapping, and the seven attachment classes; the cache stores one classification that gates and dispatch share. Channel-declared MIME leads with the transport Content-Type as a byte-arbitrated hint, normalizeMimeType folds YAML/XML synonyms so existing configured allowlists keep matching, and input_file API callers keep declared-MIME precedence over cosmetic filenames. Net -135 production LOC.

Proof: 2,153 focused tests across media-core/media-understanding/media plus boundary-contract and deadcode-mirror suites; ClawSweeper local review clean (round 5, zero findings); live Telegram default-turn probe green.

Co-authored-by: Ayaan Zaidi <hi@obviy.us>

config/knip.config.ts

@@ -602,6 +602,7 @@ const config = {
     "packages/media-core": {
       entry: [
         "src/index.ts!",
+        "src/attachment-classify.ts!",
         "src/base64.ts!",
         "src/constants.ts!",
         "src/content-length.ts!",