feat(browser): cross-platform node_path config for npx resolution

Adds [browser].node_path config option so camoufox-browser can use
an explicit npx path (essential for nvm/fnm/asdf users).

Resolution order (cross-platform):
1. config.browser.node_path — explicit user override
2. 'npx.cmd' on Windows, 'npx' on Linux/macOS — PATH lookup

Changes:
- BrowserSection config: added node_path: Option<String>
- AgentConfig: now carries browser section (was dropped in From<GyandootConfig>)
- browser_manager::init_browser(node_path): accepts optional path
- All call sites updated (run(), init_session(), subagents, tests)
- Error messages now hint at browser.node_path config

crates/gyandoot-agent/src/agent_loop.rs

@@ -193,7 +193,7 @@ impl AgentLoop {
         // ── Auto-init Camoufox and register browser tools ──────────────
         {
             let mut tool_executor = self.tool_executor.lock().await;
-            match crate::browser_manager::init_browser().await {
+            match crate::browser_manager::init_browser(self.config.browser.node_path.as_deref()).await {
                 crate::browser_manager::BrowserInitResult::Ready(client) => {
                     info!("Camoufox browser ready — registering web_search and web_browse tools");
                     crate::browser_tools::register_browser_tools(&mut tool_executor, client.clone());
@@ -400,7 +400,7 @@ impl AgentLoop {
         // Browser init
         {
             let mut tool_executor = self.tool_executor.lock().await;
-            match crate::browser_manager::init_browser().await {
+            match crate::browser_manager::init_browser(self.config.browser.node_path.as_deref()).await {
                 crate::browser_manager::BrowserInitResult::Ready(client) => {
                     info!("Camoufox browser ready — registering web_search and web_browse tools");
                     crate::browser_tools::register_browser_tools(&mut tool_executor, client.clone());
@@ -902,7 +902,7 @@ mod tests {
     use super::*;
     use gyandoot_core::config::IdentitySection;
     use gyandoot_core::{
-        config::AgentConfig, config::KanbanRootConfig, config::MemoryRootConfig,
+        config::AgentConfig, config::BrowserSection, config::KanbanRootConfig, config::MemoryRootConfig,
         config::PlatformRootConfig, config::ProviderRootConfig, config::SecurityRootConfig,
         config::SkillsRootConfig, Message, MessageRole, Role, ToolCall, ToolDefinition,
     };
@@ -921,6 +921,7 @@ mod tests {
             kanban: KanbanRootConfig::default(),
             skills: SkillsRootConfig::default(),
             identity: IdentitySection::default(),
+            browser: BrowserSection::default(),
         }
     }