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_loop.py

@@ -177,6 +177,7 @@ TOOL_SECTIONS = {
 <shell command>
 ```
 Run any shell command. Output is returned to you. Use for: installing packages, checking files, git, curl, system info, etc.
+NEVER use bash to create or change files — no `>`/`>>` redirects, no heredocs (`cat > f << 'EOF'`), no `tee`, `sed -i`, `awk -i`, no `python -c` that writes. To CREATE or fully rewrite a file use `write_file`; to change part of an existing file use `edit_file`. Those show a diff and are the ONLY allowed way to write files. (bash is for read-only inspection: `ls`, `cat` to READ, `grep`, `git status`/`git diff`, builds, installs.)
 For LONG-running commands (package installs, pip/npm, ffmpeg, model downloads, training, builds — anything that may take more than ~20s), make the FIRST line `#!bg` to run it in the BACKGROUND. You get a job id back immediately and are automatically re-invoked with the full output when it finishes — so you never block the chat waiting. Example:
 ```bash
 #!bg
@@ -220,6 +221,12 @@ Read a file and return its contents.""",
 ```
 Write content to a file. First line is the path, rest is the content.""",
 
+    "edit_file": """\
+```edit_file
+{"path": "<file path>", "old_string": "<exact text to replace>", "new_string": "<replacement>", "replace_all": false}
+```
+Edit an EXISTING file by exact string replacement. PREFER this over bash (sed/echo/redirects) for changing files — it shows a before/after diff. `old_string` must match the file exactly and be unique unless `replace_all` is true. Use write_file to create a new file.""",
+
     "create_document": """\
 ```create_document
 <title>
@@ -236,7 +243,7 @@ old text to find
 new replacement text
 <<<END>>>
 ```
-PREFERRED way to change an existing document. Find exact text and replace it. Multiple FIND/REPLACE blocks per call OK. Use this for any edit smaller than a full rewrite — adding a function, fixing a bug, tweaking a section, renaming things. **If a document is open in the editor, treat it as the user's current context: don't ask which file they mean, and don't create a new one — just edit_document the active one.** Do NOT re-send the whole file with update_document for small changes.""",
+Edit a document OPEN IN THE EDITOR PANEL — NOT a file on disk. For files on disk (home folder, project files, any real path like ~/sweden.txt) use `edit_file` instead. Find exact text and replace it. Multiple FIND/REPLACE blocks per call OK. Use for any edit smaller than a full rewrite. **If a document is open in the editor, treat it as the user's current context: don't ask which file they mean, and don't create a new one — just edit_document the active one.** Do NOT re-send the whole file with update_document for small changes.""",
 
     "update_document": """\
 ```update_document
@@ -2219,6 +2226,9 @@ async def stream_agent_loop(
             if result.get("images"):
                 img = result["images"][0]
                 tool_output_data["screenshot"] = f"data:{img['mimeType']};base64,{img['data']}"
+            # Forward a file-write diff for inline before/after rendering
+            if "diff" in result:
+                tool_output_data["diff"] = result["diff"]
             yield f'data: {json.dumps(tool_output_data)}\n\n'
 
             # Native document tools open in the editor + carry the REAL doc id.
@@ -2261,6 +2271,10 @@ async def stream_agent_loop(
             if result.get("doc_id"):
                 tool_event["doc_id"] = result["doc_id"]
                 tool_event["doc_title"] = result.get("title", "")
+            # Persist the file-write/edit diff so it re-renders on reload — without
+            # this the diff shows live but vanishes from saved history.
+            if result.get("diff"):
+                tool_event["diff"] = result["diff"]
             tool_events.append(tool_event)
             if block.tool_type in _VERIFIER_EFFECTFUL_TOOLS:
                 _effectful_used = True