feat: add inline diff previews for write actions

Show inline diffs in the CLI transcript when write_file, patch, or
skill_manage modifies files. Captures a filesystem snapshot before the
tool runs, computes a unified diff after, and renders it with ANSI
coloring in the activity feed.

Adds tool_start_callback and tool_complete_callback hooks to AIAgent
for pre/post tool execution notifications.

Also fixes _extract_parallel_scope_path to normalize relative paths
to absolute, preventing the parallel overlap detection from missing
conflicts when the same file is referenced with different path styles.

Gated by display.inline_diffs config option (default: true).

Based on PR #3774 by @kshitijk4poor.

hermes_cli/config.py

@@ -352,6 +352,7 @@ DEFAULT_CONFIG = {
         "bell_on_complete": False,
         "show_reasoning": False,
         "streaming": False,
+        "inline_diffs": True,     # Show inline diff previews for write actions (write_file, patch, skill_manage)
         "show_cost": False,       # Show $ cost in the status bar (off by default)
         "skin": "default",
         "tool_progress_command": False,  # Enable /verbose command in messaging gateway