docs(readme): collapse platform picker and optional extras into <details>

The 20+ row platform-install table and the optional-extras table (~60 lines
combined) sat between Install and the value content. Wrapping both in collapsible
<details> blocks keeps a skimmer moving to the report/commands sections while each
platform's command stays one click away. No content changed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -138,7 +138,8 @@ for example `graphify claude install --project` or `graphify codex install --pro
 
 > **Git hooks and uv tool / pipx:** `graphify hook install` embeds the current interpreter path directly into the hook scripts at install time, so the post-commit hook fires correctly even in GUI git clients and CI runners where `~/.local/bin` is not on PATH. If you reinstall or upgrade graphify, re-run `graphify hook install` to refresh the embedded path.
 
-### Pick your platform
+<details>
+<summary><b>Pick your platform</b> (20+ assistants, click to expand)</summary>
 
 | Platform | Install command |
 |----------|----------------|
@@ -172,9 +173,10 @@ Codex users also need `multi_agent = true` under `[features]` in `~/.codex/confi
 
 > Codex uses `$graphify` instead of `/graphify`.
 
-### Optional extras
+</details>
 
-Install only what you need:
+<details>
+<summary><b>Optional extras</b> (install only what you need)</summary>
 
 | Extra | What it adds | Install |
 |---|---|---|
@@ -200,6 +202,8 @@ Install only what you need:
 | `chinese` | Chinese query segmentation (jieba) | `uv tool install "graphifyy[chinese]"` |
 | `all` | Everything above | `uv tool install "graphifyy[all]"` |
 
+</details>
+
 ---
 
 ## Make your assistant always use the graph