agent: Don't track large and common binary files (#31352)

## Issue

The agent may run very slowly on projects that contain many or large
binary files not listed in `.gitignore`.


## Solution

Temporarily rewrite `.git/info/exludes` to ignore:
- Common binary files based on the extension
- Files larger than 2 MB

## Benchmark

I measure the time between sending an agent message in UI ("hitting
Enter") and actually sending it to an LLM. Ideally, it should be
instant. Numbers for a 7.7 GB Rust project with no .gitignore.

Filter                            | Time
----------------------------------|-----
No filter (= before this change)  | 62 s
Exclude common file types only    | 1.46 s
Exclude files >2MB only           | 1.16 s
Exclude both                      | 0.10 s


## Planned changes:

- [x] Exclude common binary file types
- [x] Exclude large files
- [ ] Track files added by agent so we could delete them (we can't rely
on git for that anymore)
- [ ] Don't block on waiting for a checkpoint to complete until we
really need it
- [ ] Only `git add` files that are about to change


Closes #ISSUE

Release Notes:

- Improved agent latency on repositories containing many files or large
files

crates/git/src/checkpoint.gitignore

@@ -0,0 +1,91 @@
+# This lists files that we don't track in checkpoints
+
+# Compiled source and executables
+*.exe
+*.dll
+*.so
+*.dylib
+*.a
+*.lib
+*.o
+*.obj
+*.elf
+*.out
+*.app
+*.deb
+*.rpm
+*.dmg
+*.pkg
+*.msi
+
+# Archives and compressed files
+*.7z
+*.zip
+*.tar
+*.tar.gz
+*.tgz
+*.tar.bz2
+*.tbz2
+*.tar.xz
+*.txz
+*.rar
+*.jar
+*.war
+*.ear
+
+# Media files
+*.jpg
+*.jpeg
+*.png
+*.gif
+*.ico
+*.svg
+*.webp
+*.bmp
+*.tiff
+*.mp3
+*.mp4
+*.avi
+*.mov
+*.wmv
+*.flv
+*.mkv
+*.webm
+*.wav
+*.flac
+*.aac
+
+# Database files
+*.db
+*.sqlite
+*.sqlite3
+*.mdb
+
+# Documents (often binary)
+*.pdf
+*.doc
+*.docx
+*.xls
+*.xlsx
+*.ppt
+*.pptx
+
+# IDE and editor files
+.idea/
+.vscode/
+*.swp
+*.swo
+*~
+.DS_Store
+Thumbs.db
+
+# Language-specific files
+*.rlib
+*.rmeta
+*.pdb
+*.class
+*.egg
+*.egg-info/
+*.pyc
+*.pto
+__pycache__