chore: ignore the local-only working directories every agent creates

[justinchuby]

One line of context each for three directories, measured rather than assumed.

What I found

Five entries sit at the repository root that are untracked and matched by no ignore rule:

.ort_upstream/   .squad/   .validation-worktrees/   .worktrees/   hogpids.txt

git add -A in a primary checkout stages 69 files from .validation-worktrees alone. hogpids.txt is the one I'd flag to a human reader: it has no leading dot, so in git status it reads as ordinary repository content rather than as scratch.

This is the untracked precursor to the class that has now arrived four times (.commitmsg/, .commitmsg, .pris_v4.log, .body.md). Those were all files someone created and forgot. These three are directories every agent creates by convention — throwaway worktrees for validating a PR against latest main, and a vendored upstream checkout — so unlike a filename, they are enumerable and don't require predicting anything.

This does not replace the allowlist job — verified, not assumed

@holden's Root file allowlist is the backstop and it works. I extracted the script structurally (yaml.safe_load → jobs['root-files'], asserting exactly one run: step, per the lesson from #2052) and drove it:

arm	rc
clean tree	0	Root is exactly the 36 allowlisted entr(ies).
scratch file staged, before this change	1	.validation-worktrees/ (directory)
git add -f past the new ignore rules	1	still refused, same message
only .gitignore staged (control)	0	passes

The third row is the one that matters: an ignore rule that hid a force-added file from the guard would be worse than the problem. It doesn't — ls-files reports the root segment of a tracked path regardless of ignore state. So this is a layer in front of the backstop, not a replacement for it.

Each staging arm asserts git diff --cached --name-only before believing any rc, because an acceptance arm can otherwise be satisfied by its own setup silently failing — which is exactly how a git add refused by an ignore rule produced a misleading rc=0 in #2036.

Deliberately not included

• hogpids.txt — another agent's run artifact. It should be deleted by its owner, not permanently ignored; writing a rule for it would be predicting filenames again, which is the approach chore: enumerate what belongs at the repository root #2036 retired.
• .squad/ — whether the team root is meant to be committed is a policy question, not a chore. Flagging it rather than deciding it: right now it is one git add -A away from being committed, and I don't think that's intended either way by accident. @justinchuby, worth a decision.

Root-anchored (/.worktrees/, not .worktrees/) because a crate could legitimately hold a nested fixture directory of any of these names; only the repository root is scratch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.21%. Comparing base (bb08c5a) to head (18a17fe).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2244      +/-   ##
==========================================
+ Coverage   80.64%   81.21%   +0.56%     
==========================================
  Files         432      432              
  Lines      220114   220126      +12     
  Branches   220114   220126      +12     
==========================================
+ Hits       177512   178766    +1254     
+ Misses      36717    35476    -1241     
+ Partials     5885     5884       -1     

Flag	Coverage Δ
cli-ort-linux	72.51% <ø> (ø)
cli-ort-windows	72.10% <ø> (+0.09%)	⬆️
mlas	86.02% <ø> (+0.12%)	⬆️
offline	81.35% <ø> (+0.58%)	⬆️

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see 12 files with indirect coverage changes

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