M1: read, write, bash, glob, and grep tools

Implements the first five harness-tools: read, write, bash (with timeout/output truncation), glob, and grep, plus shared path-resolution helpers. Wires them into the core tool registry and processor.

crates/harness-core/src/engine/processor.rs

@@ -424,6 +424,11 @@ impl<'a> Run<'a> {
             }
         };
 
+        // docs/05-tools.md: cap tool output at 30k chars regardless of which tool produced it.
+        let output = crate::tool::truncate::truncate(&output, &self.ctx.data_dir, &call_id)
+            .map(|t| t.text)
+            .unwrap_or(output);
+
         let final_state = if is_error {
             ToolState::Error {
                 input,

crates/harness-core/src/tool/mod.rs

@@ -122,6 +122,7 @@ pub struct ToolCtx {
     pub metadata: MetadataSink,
 }
 
+#[derive(Debug)]
 pub struct ToolOutput {
     pub title: String,
     pub output: String,