fix(extensions): say so when the staged host is too old to list vaults

A host that answers `vaults` with an error is a staged copy older than the
popup, which looked exactly like "you only have one vault" — the picker
silently fell back to its single default option. A missing host stays silent,
since pressing Fill already explains that one.

Also document that a second vault needs a ~/.obsidian-wiki/config.<name>
profile before it can appear in the list.

docs/browser-extension.md

@@ -98,6 +98,18 @@ one entry per `~/.obsidian-wiki/config.<name>`. It is the extension's equivalent
 of the `@name` override: a job application answers from `personal`, a vendor
 questionnaire from `work`, without re-pointing the symlink for everything else.
 
+A second vault only shows up once it has a profile file. Owning the folder is
+not enough — create one with `/wiki-switch new <name>`, or by hand:
+
+```bash
+printf '# My other vault\nOBSIDIAN_VAULT_PATH="%s"\n' ~/Knowledge \
+  > ~/.obsidian-wiki/config.knowledge
+```
+
+If the dropdown shows only `Default vault` when profiles do exist, the staged
+host predates this feature — re-run `install.sh` and restart Chrome. The popup
+says so explicitly rather than pretending you have one vault.
+
 The choice is remembered per browser profile, so a vault picked once stays
 picked. A profile whose `OBSIDIAN_VAULT_PATH` no longer exists is listed as
 `(missing)` and cannot be selected — and if it was the remembered one, the popup

extensions/brain/popup.js

@@ -252,16 +252,20 @@ vaultSelect.addEventListener("change", () => {
 /**
  * Ask the host which vaults exist and offer them as targets.
  *
- * Failure here is deliberately silent: the host may not be installed yet, and
- * the picker is not what the user came for. The hardcoded "Default vault"
- * option keeps Fill working exactly as it did before, and pressing Fill
- * surfaces the real host error with its install instructions.
+ * A host that is missing entirely stays silent — it may simply not be
+ * installed, and pressing Fill surfaces that with its install instructions.
+ * A host that answers but rejects the request is different: it is a staged
+ * copy older than this popup, which otherwise looks identical to "you only
+ * have one vault" and hides the picker for no visible reason.
  */
 async function loadVaults(preferred) {
   let vaults;
   try {
     const response = await chrome.runtime.sendNativeMessage(HOST_NAME, { type: "vaults" });
     vaults = response?.ok ? response.vaults : undefined;
+    if (!response?.ok) {
+      setStatus("Vault list unavailable — re-run host/install.sh, then ⌘Q and reopen Chrome.", "error");
+    }
   } catch {
     vaults = undefined;
   }