Fix UTF-8 slicing panics: use floor_char_boundary for all truncation

Byte-position truncation (&s[..s.len().min(N)]) panics when position
N lands inside a multi-byte character. Fixed in parser debug logging,
API error messages, oneshot response logging, and CLI agent display.

Also fixed tool dispatch permissions (removed global fallback).

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

src/agent/oneshot.rs

@@ -179,7 +179,7 @@ impl AutoAgent {
             }
 
             dbglog!("[auto] {} response: {}",
-                self.name, &text[..text.len().min(200)]);
+                self.name, &text[..text.floor_char_boundary(text.len().min(200))]);
 
             if next_step < self.steps.len() {
                 if let Some(ref check) = bail_fn {