fix(funnel): repair activation-path bugs found in end-to-end audit (#114)

Systematic trace of the activation funnel (store listing -> docs setup
guide -> in-app onboarding -> connect flow) after #113 showed a docs
404 bug had slipped through. Fixes found in this pass:

- README.md and CONTRIBUTING.md still told developers to run
  `npm install -g opencode` (missing -ai), the exact 404 #113 fixed
  everywhere else. Same package-name fix applied here.
- src/lib/diagnostics.ts: the health probe reported ok:true for any
  resolved fetch, including 401/403/404/500 responses. A wrong
  password or broken server therefore classified as "Health endpoint
  responded - connection actually works now.", contradicting the
  error shown right next to it. Health now requires a real 2xx;
  root-reachability probe explicitly opts out (requireOk: false) to
  keep its existing reachability-only semantics (already encoded in
  diagnostics-classify.test.ts).
- docs-site/vs-termux: architecture explainer described
  /session/{id}/events and /session/{id}/chat, neither of which
  exists; the app actually uses /global/event and
  /session/{id}/prompt_async (src/lib/sdk.ts).
- docs-site/features: opencode repo link pointed at
  github.com/opencode-ai/opencode instead of github.com/sst/opencode,
  the org used everywhere else in the docs.
- Stale "v0.4.3" version strings (README x3, docs-site/download x2)
  bumped to v0.4.7, the actual latest shipped release confirmed live
  on GitHub Releases and the F-Droid repo index. Fixed the associated
  Android min-OS contradiction (7.0 vs 8.0+) to 7.0, matching the live
  F-Droid manifest's minSdkVersion 24.
- Dropped stale "GPT-4" model naming on 3 docs-site pages, consistent
  with the model-agnostic policy already applied to the Play listing
  in #83.

See PR body for the found-not-fixed list (Play Console listing drift,
GitHub Sponsors not enabled, a Cloud-vs-waitlist messaging conflict)
that need a product/human decision rather than a code fix.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

docs-site/features/index.html

@@ -50,7 +50,7 @@
   "offers": { "@type": "Offer", "price": "0", "priceCurrency": "USD" },
   "featureList": [
     "Real-time AI coding sessions via Server-Sent Events",
-    "Multi-model support: Claude, GPT-4, Gemini, Ollama, and any opencode-compatible model",
+    "Multi-model support: Claude, GPT, Gemini, Ollama, and any opencode-compatible model",
     "Tailscale and VPN connectivity for private server access",
     "Streaming diff and tool-use output as the agent works",
     "Full session history with scroll and search",
@@ -234,7 +234,7 @@ footer a:hover{color:var(--accent)}
     <h2>System requirements</h2>
     <ul class="check-list">
       <li>Android 7.0 (API 24) or later</li>
-      <li><a href="https://github.com/opencode-ai/opencode">opencode</a> running on a reachable server (<code>opencode serve</code>)</li>
+      <li><a href="https://github.com/sst/opencode">opencode</a> running on a reachable server (<code>opencode serve</code>)</li>
       <li>Network path from phone to server (LAN, Tailscale VPN, or any tunnel)</li>
       <li>~15 MB storage for the app; session history grows with use</li>
     </ul>