fix(websearch): detect Gemini CLI auth status before showing Ready

Previously, WebSearch status showed "Ready (Gemini)" when the CLI binary
was installed, misleading users who hadn't authenticated yet.

Changes:
- Add isGeminiAuthenticated() to check ~/.gemini/oauth_creds.json
- Add 'needs_auth' state to WebSearchReadiness type
- Show warning "[!] Gemini: run 'gemini' to login" when not authenticated
- Fix incorrect 'gemini auth login' references (no such command exists)
- Update docs with correct npm install and authentication instructions

lib/hooks/websearch-transformer.cjs

@@ -513,8 +513,8 @@ function outputError(query, error, providerName) {
     `Query: "${query}"`,
     '',
     'Troubleshooting:',
-    '  - Check if Gemini CLI is authenticated: gemini auth status',
-    '  - Re-authenticate if needed: gemini auth login',
+    '  - Check ~/.gemini/oauth_creds.json exists',
+    '  - Re-authenticate: run `gemini` (opens browser)',
   ].join('\n');
 
   const output = {
@@ -546,7 +546,7 @@ function outputNoProvidersEnabled(query) {
     '',
     '1. Gemini CLI (FREE, 1000 req/day):',
     '   npm install -g @google/gemini-cli',
-    '   gemini auth login',
+    '   gemini  # opens browser to authenticate',
     '',
     '2. OpenCode (FREE via Zen):',
     '   curl -fsSL https://opencode.ai/install | bash',
@@ -595,7 +595,7 @@ function outputAllFailedMessage(query, errors) {
     `Query: "${query}"`,
     '',
     'Troubleshooting:',
-    '  - Gemini: gemini auth status / gemini auth login',
+    '  - Gemini: run `gemini` to authenticate (opens browser)',
     '  - OpenCode: opencode --version',
     '  - Grok: Check XAI_API_KEY environment variable',
   ].join('\n');