feat: Add edit_file tool + file-change diffs (#1239)

* Add edit_file tool + file-change diffs

edit_file is an exact old_string -> new_string replacement on a file on disk
(fails if old_string is missing or non-unique unless replace_all); write_file
also returns a unified diff. Diffs render collapsed in the tool bubble
(filename + +adds/-dels, theme colors); the raw JSON command box is hidden.

Security: edit_file is a sensitive filesystem-write tool, treated everywhere
write_file is —
  - added to NON_ADMIN_BLOCKED_TOOLS (is_public_blocked_tool / blocked_tools_for_owner),
    so on auth-enabled deployments a non-admin cannot run it; execute_tool_block
    refuses it for non-admin owners.
  - confined by the same path policy as read_file/write_file (allowlist +
    sensitive-file deny) via _resolve_tool_path.

Disambiguation in tool descriptions + bash prompt: edit_file/write_file are the
only way to write files (they show a diff) — never edit_document (editor panel)
or a bash heredoc/redirect.

Tests (tests/test_edit_file.py): non-admin block (policy + execution gate),
successful edit, not-found old_string, non-unique old_string (+ replace_all),
and path outside the allowed roots.

Files: src/tool_execution.py, src/agent_loop.py, src/tool_schemas.py,
src/agent_tools.py, src/tool_index.py, static/js/chat.js, static/style.css,
tests/test_edit_file.py.

* Drop redundant import os in write_file closure

os is already imported at module top.

src/agent_tools.py

@@ -26,7 +26,7 @@ MAX_OUTPUT_CHARS = 10_000
 MAX_READ_CHARS = 20_000
 
 # Tool types that trigger execution
-TOOL_TAGS = {"bash", "python", "web_search", "web_fetch", "read_file", "write_file",
+TOOL_TAGS = {"bash", "python", "web_search", "web_fetch", "read_file", "write_file", "edit_file",
              "create_document", "update_document", "edit_document",
              "search_chats",
              "chat_with_model", "create_session", "list_sessions",