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/path_search_tool/description.md

@@ -1,3 +1,7 @@
-Returns paths in the project which match the given glob.
+Fast file pattern matching tool that works with any codebase size
 
-Results are paginated with 50 matches per page. Use the optional 'offset' parameter to request subsequent pages.
+- Supports glob patterns like "**/*.js" or "src/**/*.ts"
+- Returns matching file paths sorted alphabetically
+- Prefer the `regex_search` tool to this tool when searching for symbols unless you have specific information about paths.
+- Use this tool when you need to find files by name patterns
+- Results are paginated with 50 matches per page. Use the optional 'offset' parameter to request subsequent pages.