Add pre-commit hook for autoformatting

[cyrus-]

Summary

• Adds a shared pre-commit hook (under scripts/git-hooks/) that runs dune fmt --auto-promote and re-stages any originally-staged files the formatter touched, so commits never contain unformatted code.
• Files that weren't staged are deliberately left alone — unrelated unstaged WIP won't be swept into the commit.
• make install-hooks sets core.hooksPath to scripts/git-hooks. It's idempotent and is now a dependency of deps, dev, and dev-student, so anyone running setup or a normal build picks up the hook automatically (no manual opt-in needed).
• git commit --no-verify still bypasses the hook for cases where it's necessary.

Test plan

• make install-hooks (first run installs, second run is silent — verified)
• Commit with no formatting changes — hook runs, commit goes through
• Stage a misformatted file with no other unstaged changes — formatted version lands in commit
• Stage a misformatted file while having unrelated unstaged edits in another file — only the staged file is in the commit (formatted); unrelated WIP stays unstaged
• git commit --no-verify — bypasses the hook
• Reviewer: try make deps once on an existing clone to confirm hooks auto-install

🤖 Generated with Claude Code

[7h3kk1d]

This is going to behave interestingly in the case where someone partially stages a file. Resulting in the entire file being committed.

[7h3kk1d]

Having thought about this more my personal preference is probably to just abort the commit if there's reformatting. So I would probably stash the unstaged components run formatting (abort if there's changes) then commit then unstash.

[7h3kk1d]

Alternatively we can have github reformat on the server

[7h3kk1d]

@Negabinary can you try this with vs code + windows. Not sure how it'll work there.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.39%. Comparing base (64e7def) to head (e4396c2).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #2233      +/-   ##
==========================================
- Coverage   50.42%   50.39%   -0.04%     
==========================================
  Files         296      296              
  Lines       39452    39452              
==========================================
- Hits        19894    19880      -14     
- Misses      19558    19572      +14     

see 16 files with indirect coverage changes

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