Skip JJ-INSTRUCTIONS detection if directory

[darkfeline]

The JJ-INSTRUCTIONS file is documented to be a file, so ignore directories with matching names.

The context for this is that we use a FUSE workspace manager for development. I can't share all the details, but basically the FUSE mount automagically generates workspace directories like:

.../mount/workspace1
.../mount/workspace2
.../mount/JJ-INSTRUCTIONS

While this use case is a bit specific, I don't see any harm tightening the check here in majutsu.

Summary by CodeRabbit

• Bug Fixes
  • Fixed diffedit root directory detection to properly validate that JJ-INSTRUCTIONS is a regular file, preventing incorrect root detection when a directory with the same name exists.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 4bc8b1b8-3b6c-4ce3-be3e-41771362212d

📥 Commits

Reviewing files that changed from the base of the PR and between 97169be and 2e4c45b.

📒 Files selected for processing (2)

• majutsu-edit.el
• test/majutsu-edit-test.el

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Walkthrough

majutsu-edit--diffedit-root is updated to use a predicate lambda with file-regular-p when calling locate-dominating-file, ensuring JJ-INSTRUCTIONS must be a regular file to qualify as a root marker. A new ERT test covers the case where JJ-INSTRUCTIONS is a directory.

Changes

Diffedit root regular-file predicate

Layer / File(s)	Summary
Root detection predicate and regression test majutsu-edit.el, test/majutsu-edit-test.el	majutsu-edit--diffedit-root switches from a bare sentinel name to a lambda that checks file-regular-p on JJ-INSTRUCTIONS; the accompanying ERT test constructs a JJ-INSTRUCTIONS directory and asserts the function returns nil for a nested path.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

A sentinel once walked without a care,
"Is it a file?" asked the rabbit with flair.
file-regular-p sniffs the trail,
Directories? No — those shall fail!
🐇 Only true files mark the root with style.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly and concisely summarizes the main change: skipping JJ-INSTRUCTIONS when it's a directory rather than a file.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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