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-subagents/src/lib.rs

@@ -268,6 +268,7 @@ impl SubAgentWorker {
             kanban: gyandoot_core::KanbanRootConfig::default(),
             skills: gyandoot_core::SkillsRootConfig::default(),
             identity: gyandoot_core::IdentitySection::default(),
+            browser: gyandoot_core::BrowserSection::default(),
         };
 
         // Try to create the agent loop. If the provider config isn't viable