chore: allow local justfile overrides

[Shgit29]

Description

Closes #401

Adds support for local justfile overrides by including an optional justfile.local import at the top of the repository justfiles.

This allows contributors to define their own local recipes without modifying tracked files. The import uses import?, so it remains optional and does not affect existing workflows if the file is absent.

Also adds justfile.local to .gitignore to prevent accidental commits.

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Notes to the reviewers

Followed the Just optional import feature as documented:
https://just.systems/man/en/imports.html

The change is intentionally minimal and only adds the optional import without modifying any existing recipes or behavior.

Applies the change to both root and examples justfiles for consistency.

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Changelog notice

Added support for optional local justfile overrides via justfile.local.

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Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[thunderbiscuit]

Tested ACK 68dccaf. Works great!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.21%. Comparing base (35a87f3) to head (68dccaf).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #460   +/-   ##
=======================================
  Coverage   80.21%   80.21%           
=======================================
  Files          24       24           
  Lines        5348     5348           
  Branches      242      242           
=======================================
  Hits         4290     4290           
  Misses        980      980           
  Partials       78       78           

Flag	Coverage Δ
rust	80.21% <ø> (ø)

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[ValuedMammal]

ACK 68dccaf