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/subconscious/defs.rs

@@ -606,7 +606,7 @@ fn resolve_conversation(budget: Option<usize>) -> String {
         if total_bytes >= max_bytes { break; }
         let name = if role == "user" { &cfg.user_name } else { &cfg.assistant_name };
         let formatted = if !ts.is_empty() {
-            oldest_ts = ts[..ts.len().min(19)].to_string();
+            oldest_ts = ts[..ts.floor_char_boundary(ts.len().min(19))].to_string();
             format!("**{}** {}: {}", name, &oldest_ts, content)
         } else {
             format!("**{}:** {}", name, content)