feat(helexa-acp): add tools, session modes, and permission gating

Stage 3 introduces five tools (read_file, write_file, edit_file,
list_dir, bash) backed by ACP fs/* and terminal/* calls, a
ClientOps trait so the runner is mock-testable, two session modes
(default + bypassPermissions) with session/set_mode honouring them,
and a tool-call loop in the agent that streams the model, dispatches
each call, feeds results back into history, and re-enters until the
model finishes or MAX_TOOL_ROUNDS is hit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/helexa-acp/src/main.rs

@@ -20,6 +20,8 @@ mod config;
 mod prompt;
 mod provider;
 mod session;
+mod tool_runner;
+mod tools;
 
 use agent::Agent;
 use config::{Config, EndpointConfig, WireApi};