Systematically optimize agentic editing performance (#28961)

Now that we've established a proper eval in tree, this PR is reboots of
our agent loop back to a set of minimal tools and simpler prompts. We
should aim to get this branch feeling subjectively competitive with
what's on main and then merge it, and build from there.

Let's invest in our eval and use it to drive better performance of the
agent loop. How you can help: Pick an example, and then make the outcome
faster or better. It's fine to even use your own subjective judgment, as
our evaluation criteria likely need tuning as well at this point. Focus
on making the agent work better in your own subjective experience first.
Let's focus on simple/practical improvements to make this thing work
better, then determine how we can craft our judgment criteria to lock
those improvements in.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Agus <agus@zed.dev>
Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Michael Sloan <mgsloan@gmail.com>

crates/assistant_tools/src/edit_file_tool/description.md

@@ -0,0 +1,45 @@
+This is a tool for editing files. For moving or renaming files, you should generally use the `terminal` tool with the 'mv' command instead. For larger edits, use the `create_file` tool to overwrite files.
+
+Before using this tool:
+
+1. Use the `read_file` tool to understand the file's contents and context
+
+2. Verify the directory path is correct (only applicable when creating new files):
+   - Use the `list_directory` tool to verify the parent directory exists and is the correct location
+
+To make a file edit, provide the following:
+1. path: The full path to the file you wish to modify in the project. This path must include the root directory in the project.
+2. old_string: The text to replace (must be unique within the file, and must match the file contents exactly, including all whitespace and indentation)
+3. new_string: The edited text, which will replace the old_string in the file.
+
+The tool will replace ONE occurrence of old_string with new_string in the specified file.
+
+CRITICAL REQUIREMENTS FOR USING THIS TOOL:
+
+1. UNIQUENESS: The old_string MUST uniquely identify the specific instance you want to change. This means:
+   - Include AT LEAST 3-5 lines of context BEFORE the change point
+   - Include AT LEAST 3-5 lines of context AFTER the change point
+   - Include all whitespace, indentation, and surrounding code exactly as it appears in the file
+
+2. SINGLE INSTANCE: This tool can only change ONE instance at a time. If you need to change multiple instances:
+   - Make separate calls to this tool for each instance
+   - Each call must uniquely identify its specific instance using extensive context
+
+3. VERIFICATION: Before using this tool:
+   - Check how many instances of the target text exist in the file
+   - If multiple instances exist, gather enough context to uniquely identify each one
+   - Plan separate tool calls for each instance
+
+WARNING: If you do not follow these requirements:
+   - The tool will fail if old_string matches multiple locations
+   - The tool will fail if old_string doesn't match exactly (including whitespace)
+   - You may change the wrong instance if you don't include enough context
+
+When making edits:
+   - Ensure the edit results in idiomatic, correct code
+   - Do not leave the code in a broken state
+   - Always use fully-qualified project paths (starting with the name of one of the project's root directories)
+
+If you want to create a new file, use the `create_file` tool instead of this tool. Don't pass an empty `old_string`.
+
+Remember: when making multiple file edits in a row to the same file, you should prefer to send all edits in a single message with multiple calls to this tool, rather than multiple messages with a single call each.