tools: add cd tool for changing working directory

Uses std::env::set_current_dir() syscall so the change affects
all subsequent tool invocations. Supports absolute paths, relative
paths, and ~ expansion.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/tools/cd.rs

@@ -0,0 +1,39 @@
+use std::sync::Arc;
+use std::path::PathBuf;
+
+// tools/cd.rs — Change working directory
+//
+// Uses the chdir syscall so it affects all tools.
+
+pub fn tool() -> super::Tool {
+    super::Tool {
+        name: "cd",
+        description: "Change the current working directory.",
+        parameters_json: r#"{"type":"object","properties":{"path":{"type":"string","description":"The directory to change to (absolute or relative)"}},"required":["path"]}"#,
+        handler: Arc::new(|_agent, v| Box::pin(async move {
+            let path = v.get("path").and_then(|v| v.as_str())
+                .ok_or_else(|| anyhow::anyhow!("'path' parameter is required"))?;
+            if path.is_empty() { anyhow::bail!("'path' parameter cannot be empty"); }
+
+            // Resolve ~ to home directory
+            let resolved = if path.starts_with('~') {
+                let home = dirs::home_dir()
+                    .ok_or_else(|| anyhow::anyhow!("could not determine home directory"))?;
+                home.join(path.strip_prefix("~/").unwrap_or(path))
+            } else {
+                PathBuf::from(path)
+            };
+
+            // Change directory (this is the actual chdir syscall)
+            std::env::set_current_dir(&resolved)
+                .map_err(|e| anyhow::anyhow!("cd: {}: {}", path, e))?;
+
+            // Return the canonical path
+            let canonical = std::env::current_dir()
+                .map(|p| p.display().to_string())
+                .unwrap_or_else(|_| resolved.display().to_string());
+
+            Ok(canonical)
+        })),
+    }
+}

src/agent/tools/mod.rs

@@ -6,13 +6,14 @@
 
 // Core tools
 mod ast_grep;
-pub mod lsp;
-pub mod mcp_client;
 mod bash;
+mod cd;
 pub mod channels;
 mod edit;
 mod glob;
 mod grep;
+pub mod lsp;
+pub mod mcp_client;
 pub mod memory;
 mod read;
 mod web;
@@ -177,7 +178,7 @@ pub async fn dispatch_with_agent(
 pub fn tools() -> Vec<Tool> {
     let mut all = vec![
         read::tool(), write::tool(), edit::tool(),
-        grep::tool(), glob::tool(), bash::tool(),
+        grep::tool(), glob::tool(), bash::tool(), cd::tool(),
         ast_grep::tool(), vision::tool(),
     ];
     all.extend(web::tools());