feat(setup): auto-detect npx path during installation

The setup wizard now auto-detects the npx/npx.cmd path via
`which npx` (Unix) or `where npx.cmd` (Windows) and writes it
to [browser].node_path in config.toml.

Changes:
- detect_npx_path(): cross-platform detection using which/where
- SerializedConfig now includes optional [browser] section
- Flat config parser (main.rs) handles [browser] section
- browser: BrowserSection threaded through AgentConfig everywhere
- init_browser() accepts node_path parameter with proper fallback

src/main.rs

@@ -293,6 +293,8 @@ fn load_and_convert_config(path: &std::path::Path) -> Result<gyandoot_core::conf
     struct FlatConfig {
         provider: FlatProvider,
         agent: Option<FlatAgent>,
+        #[serde(default)]
+        browser: Option<FlatBrowser>,
     }
 
     #[derive(serde::Deserialize)]
@@ -316,6 +318,12 @@ fn load_and_convert_config(path: &std::path::Path) -> Result<gyandoot_core::conf
         name: Option<String>,
     }
 
+    #[derive(serde::Deserialize)]
+    struct FlatBrowser {
+        #[serde(default)]
+        node_path: Option<String>,
+    }
+
     let flat: FlatConfig = toml::from_str(&content)?;
     tracing::info!("Loaded config from {} (converted from flat [provider] format)", path.display());
 
@@ -346,7 +354,10 @@ fn load_and_convert_config(path: &std::path::Path) -> Result<gyandoot_core::conf
         platform: gyandoot_core::config::PlatformSection::default(),
         memory: gyandoot_core::config::MemorySection::default(),
         gateway: gyandoot_core::config::GatewaySection::default(),
-        browser: gyandoot_core::config::BrowserSection::default(),
+        browser: gyandoot_core::config::BrowserSection {
+            node_path: flat.browser.and_then(|b| b.node_path),
+            ..gyandoot_core::config::BrowserSection::default()
+        },
     };
 
     Ok(gyandoot_core::config::AgentConfig::from(gyandoot_cfg))