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-core/src/config.rs

@@ -230,6 +230,12 @@ pub struct BrowserSection {
     /// Camofox URL for browser automation.
     #[serde(default)]
     pub camofox_url: Option<String>,
+    /// Explicit path to the npx (or npx.cmd on Windows) executable.
+    /// When set, `init_browser()` uses this instead of bare `"npx"`.
+    /// Useful when nvm / fnm / asdf manages Node — e.g.
+    /// `/home/user/.nvm/versions/node/v22/bin/npx`.
+    #[serde(default)]
+    pub node_path: Option<String>,
 }
 
 impl Default for BrowserSection {
@@ -237,6 +243,7 @@ impl Default for BrowserSection {
         Self {
             enabled: false,
             camofox_url: None,
+            node_path: None,
         }
     }
 }
@@ -292,6 +299,7 @@ pub struct AgentConfig {
     pub kanban: KanbanRootConfig,
     pub skills: SkillsRootConfig,
     pub identity: IdentitySection,
+    pub browser: BrowserSection,
 }
 
 impl Default for AgentConfig {
@@ -309,6 +317,7 @@ impl Default for AgentConfig {
             kanban: KanbanRootConfig::default(),
             skills: SkillsRootConfig::default(),
             identity: IdentitySection::default(),
+            browser: BrowserSection::default(),
         }
     }
 }
@@ -348,6 +357,7 @@ impl From<GyandootConfig> for AgentConfig {
             kanban: KanbanRootConfig::default(),
             skills: SkillsRootConfig::default(),
             identity: cfg.identity,
+            browser: cfg.browser,
         }
     }
 }