Load a config that has no age recipient (closes #221)

The README's steps for restoring on another machine failed at the first
command: `config init` wrote a placeholder recipient, and `config.Load`
rejects any recipient that does not parse. `config init` now writes an
empty `age_recipients` list, `config.Load` accepts an empty list, and
`snapshot create` refuses to start without a recipient. A malformed
recipient is still rejected at load.

The recovery-host test now builds its config with `config init` and
`config set` and reads it through `config.Load`, so it imports
`internal/cli`. On a fresh file, `config set age_recipients.0` writes the
list in flow style (`[age1...]`).

Model: opus-5-5

internal/cli/config.go

@@ -45,16 +45,19 @@ const defaultConfigTemplate = `# vaultik configuration
 
 # ─── REQUIRED ────────────────────────────────────────────────────────────────
 
-# Age recipient public keys for encryption.
+# Age recipient public keys for encryption. snapshot create needs at least
+# one; listing, verifying and restoring do not, so a machine that only
+# restores can leave this empty.
 # Backups are encrypted to ALL listed recipients; any one of the corresponding
 # private keys can decrypt. Adding a recipient later does not re-encrypt data
 # already stored: deduplicated chunks and existing blobs stay encrypted to the
 # earlier recipients, so a newly added key cannot restore them on its own (see
-# docs/REPOSTRUCTURE.md, Accepted Risks). Generate a keypair with:
+# docs/REPOSTRUCTURE.md, Accepted Risks). Generate a keypair and add its
+# public key with:
 #   age-keygen -o vaultik_backup_private_key.txt
 #   grep 'public key' vaultik_backup_private_key.txt
-age_recipients:
-  - age1REPLACE_WITH_YOUR_PUBLIC_KEY
+#   vaultik config set age_recipients.0 age1...
+age_recipients: []
 
 # Named snapshots. Each snapshot backs up one or more paths and can have its
 # own exclude patterns in addition to the global excludes below.