docs(changelog): backup & restore (Plan 3A)

CHANGELOG.md

@@ -28,6 +28,35 @@
 - **`relicario status`** — vault summary: root path, item count
   (active / trashed), attachment count + total bytes, registered device
   count, last commit (`%h %s`).
+- **Backup & restore.** New `relicario backup export <out.relbak>` and
+  `relicario backup restore <in.relbak> [<dir>]` commands. The `.relbak`
+  format is a single encrypted file: Argon2id-derived key from a
+  user-chosen backup passphrase (independent of the vault factor),
+  XChaCha20-Poly1305 ciphertext, zstd-compressed JSON envelope.
+  Reference image and `.git/` history are opt-in inclusions
+  (`--include-image`, `--no-history`).
+- **Vault-tab Backup & Restore panel.** Export downloads the
+  `.relbak` via `chrome.downloads`. Restore takes a file + backup
+  passphrase + new-remote config and writes the vault into a fresh
+  empty repo (refuses to clobber existing). Git history is never
+  bundled from the extension — CLI is the source of full backups.
+- **`relicario status` shows last export age.** New `Last export:
+  <human-readable>` line reading `.relicario/last_backup` (a marker
+  file `cmd_backup_export` writes on success). Reads "never" for
+  fresh vaults, "4 days ago" otherwise.
+
+### Known limitations
+
+- **Mid-restore failure leaves the target remote in a half-written
+  state.** `cmd_backup_restore` and the vault-tab Restore panel both
+  write artifacts sequentially via `writeFileCreateOnly`. If the
+  process is interrupted partway, a retry against the same remote
+  refuses to clobber. Workaround: delete the partial repo and retry.
+- **Cross-tool backup compatibility.** CLI-exported backups stored
+  attachments at `<item_id>/<aid>.enc`; extension stores at flat
+  `<aid>.bin`. The `.relbak` envelope canonicalizes to `<item_id>/<aid>`
+  keys and each tool translates at the boundary. Round-trip works in
+  both directions.
 
 ### Internal