feat(cli): cmd_backup_restore — unpack .relbak into target dir

Refuses non-empty target, prompts for backup passphrase, writes the
full vault layout, untars .git/ when bundled or git-inits a fresh
'restore from backup <iso8601>' commit otherwise.

Also tightens error context on tar_directory's builder.finish().

crates/relicario-cli/src/main.rs

@@ -1440,13 +1440,103 @@ fn tar_directory(dir: &std::path::Path) -> Result<Vec<u8>> {
         let mut builder = tar::Builder::new(&mut buf);
         builder.append_dir_all(".", dir)
             .with_context(|| format!("failed to tar {}", dir.display()))?;
-        builder.finish()?;
+        builder.finish().with_context(|| "failed to finalize git tar")?;
     }
     Ok(buf)
 }
 
-fn cmd_backup_restore(_input: PathBuf, _target: PathBuf) -> Result<()> {
-    anyhow::bail!("cmd_backup_restore not yet implemented")
+fn cmd_backup_restore(input: PathBuf, target: PathBuf) -> Result<()> {
+    use std::fs;
+    use relicario_core::backup;
+    use zeroize::Zeroizing;
+
+    let target = if target.is_absolute() {
+        target
+    } else {
+        std::env::current_dir()?.join(&target)
+    };
+
+    if target.join(".relicario").exists() {
+        anyhow::bail!(
+            "target dir already contains a relicario vault; restore refuses to overwrite — use an empty directory: {}",
+            target.display()
+        );
+    }
+    fs::create_dir_all(&target)
+        .with_context(|| format!("failed to create target {}", target.display()))?;
+
+    // Read input file.
+    let bytes = fs::read(&input)
+        .with_context(|| format!("failed to read backup file {}", input.display()))?;
+
+    // Backup passphrase prompt.
+    let passphrase = if let Ok(p) = std::env::var("RELICARIO_TEST_BACKUP_PASSPHRASE") {
+        Zeroizing::new(p)
+    } else {
+        Zeroizing::new(rpassword::prompt_password("Backup passphrase: ")?)
+    };
+
+    let unpacked = backup::unpack_backup(&bytes, &passphrase)
+        .map_err(|e| match e {
+            relicario_core::RelicarioError::Decrypt =>
+                anyhow::anyhow!("wrong backup passphrase, or the file is corrupt"),
+            other => anyhow::anyhow!(other),
+        })?;
+
+    // Write vault layout.
+    let relicario_dir = target.join(".relicario");
+    fs::create_dir_all(&relicario_dir)?;
+    fs::create_dir_all(target.join("items"))?;
+    fs::create_dir_all(target.join("attachments"))?;
+
+    fs::write(relicario_dir.join("salt"), unpacked.salt)?;
+    fs::write(relicario_dir.join("params.json"), &unpacked.params_json)?;
+    fs::write(relicario_dir.join("devices.json"), &unpacked.devices_json)?;
+    fs::write(target.join("manifest.enc"), &unpacked.manifest_enc)?;
+    fs::write(target.join("settings.enc"), &unpacked.settings_enc)?;
+
+    for item in &unpacked.items {
+        fs::write(target.join("items").join(format!("{}.enc", item.id)), &item.ciphertext)?;
+    }
+    for a in &unpacked.attachments {
+        let dir = target.join("attachments").join(&a.item_id);
+        fs::create_dir_all(&dir)?;
+        fs::write(dir.join(format!("{}.enc", a.attachment_id)), &a.ciphertext)?;
+    }
+
+    // Reference image (if present).
+    if let Some(jpg) = &unpacked.reference_jpg {
+        let path = target.join("reference.jpg");
+        fs::write(&path, jpg)
+            .with_context(|| format!("failed to write reference image {}", path.display()))?;
+    }
+
+    // .git/ history.
+    if let Some(tar_bytes) = &unpacked.git_archive {
+        let mut archive = tar::Archive::new(tar_bytes.as_slice());
+        archive.unpack(target.join(".git"))
+            .with_context(|| "failed to untar .git/")?;
+    } else {
+        // No history bundled — start a fresh git repo.
+        let status = crate::helpers::git_command(&target, &["init"]).status()?;
+        if !status.success() { anyhow::bail!("git init failed"); }
+
+        // .gitignore — exclude reference image if present.
+        if target.join("reference.jpg").exists() {
+            fs::write(target.join(".gitignore"), "reference.jpg\n")?;
+        }
+
+        let _ = crate::helpers::git_command(&target, &["add", "."]).status()?;
+        let now_iso = crate::helpers::iso8601(relicario_core::now_unix());
+        let msg = format!("restore from backup {now_iso}");
+        let _ = crate::helpers::git_command(&target, &["commit", "-m", &msg]).status()?;
+    }
+
+    eprintln!(
+        "Restored vault to {}. Unlock with your passphrase + reference image.",
+        target.display()
+    );
+    Ok(())
 }
 
 fn cmd_trash_empty() -> Result<()> {