Adopt local vaults as retained hosted mirrors

[callumalpass]

Summary

• add explicit portable authority adoption for an existing local mdbase vault
• upload configuration, types, configured Bases views, and Markdown as one canonical snapshot
• persist the one-time credential in Obsidian SecretStorage and the resumable checkpoint/snapshot under .mdbase
• make hosted mdbase authoritative only after the exact fenced snapshot activates
• retain the existing vault as a pre-approved read-write mirror and fall back to owner enrollment if that pairing response is interrupted
• block plugin authority writes while activation is unresolved, while keeping later external edits local for the new mirror

Verification

• npm test (38 passed)
• interruption coverage: lost activation response/restart, interrupted final upload, post-fence edit, cancellation, and expired checkpoint
• mirror coverage: retained pairing, read/write convergence, interrupted mirror write, conflicts/collisions, reserved paths
• npm run check:mobile (raw 512,524 bytes; gzip 149,959 bytes, both within budget)
• npm run build
• git diff --check

SDK artifacts are vendored from merged mdbase-connect commit c3a44aa7063a7bae1bbf89bbc8c5d7e868a95119.