omni-git guest-fs bakery dedup (diskforge wiring)

_add_image is ADDITIVE: whole image still stored byte-for-byte as one content-addressed blob (pull byte-identical) PLUS the guest filesystem is walked by the diskforge usermode reader (import diskforge.wire, no mount/loopback/qemu-nbd/kernel); each guest file is content-addressed and deduped into the store (RFC-0009: sha256 is authoritative identity). Sets manifest["members_via"]="diskforge". Degrades to stock on ImportError/MemberIndexError.

BAKED IN: the diskforge package is VENDORED at the repo root (importable as diskforge, beside omni/), so a fresh clone auto-activates it with NO PYTHONPATH hack or external checkout. Vendored verbatim from testmonkeyalpha-group/jazz-forge/diskforge @ 8e6064a (see diskforge/_VENDOR.txt).

Verified: import diskforge.wire resolves to the vendored copy with ONLY the repo root on path; omni selftest 254/254 (0 fail) with external diskforge REMOVED from PYTHONPATH. Canonical main = e0bb706 (round 100 + wiring + vendored), pushed to GitLab testmonkeyalpha/omni-git-entry-claude. Tracked by congruency #493; depends on diskforge #495.