Add Python 3.14 support by forcing parallelism to use fork

[forsyth2]

Summary

Objectives:

• Adds support for Python 3.14
• Make simple parallelism fix to support Python 3.14. More robust changes may be introduced later: see Design decisions for updating parallelism in `zstash` #430.

Issue resolution:

• Resolves the need for Python 3.14 support (no issue created)
• Cherry-picks relevant commits from Fix Python 3.14+ parallelism: force fork start method and fix tar.extract filter #429

Select one: This pull request is...

• a bug fix: increment the patch version
• a small improvement: increment the minor version
• a new feature: increment the minor version
• an incompatible (non-backwards compatible) API change: increment the major version

Small Change

• To merge, I will use "Squash and merge". That is, this change should be a single commit.
• Logic: I have visually inspected the entire pull request myself.
• Pre-commit checks: All the pre-commits checks have passed.

[forsyth2]

Created this PR from #429 via the following steps

Steps to cherry-pick relevant commits

cd ~/ez/zstash
# On branch copilot/force-fork-parallelism
# nothing to commit, working tree clean
git log --oneline | head -n 8
# 1496e42 Trigger GitHub Actions
# cb558ab Fix pre-commit checks
# c70679a Fix tar.extract filter for Python 3.12+ to allow absolute symlinks
# 0baaa86 Force fork start method for multiprocessing (Python 3.14+)
# 877660d Initial plan
# 9bdbda4 Address review comments
# 282def2 Add Python 3.14 support
# c14b8ee Add AGENTS.md (#423)

git fetch upstream main
git checkout -b force-fork-parallel upstream/main
git fetch upstream copilot/force-fork-parallelism
git cherry-pick 282def2 # Add Python 3.14 support
git cherry-pick 9bdbda4 # Address review comments
# Skip 877660d Initial plan
git cherry-pick 0baaa86 # Force fork start method for multiprocessing (Python 3.14+)
git cherry-pick c70679a # Fix tar.extract filter for Python 3.12+ to allow absolute symlinks
git cherry-pick cb558ab # Fix pre-commit checks
# Skip 1496e42 Trigger GitHub Actions

git log --oneline | head -n 6
# f5a8e4c Fix pre-commit checks
# 2703bf6 Fix tar.extract filter for Python 3.12+ to allow absolute symlinks
# d37c78f Force fork start method for multiprocessing (Python 3.14+)
# 60b435d Address review comments
# 7d048f2 Add Python 3.14 support
# c14b8ee Add AGENTS.md (#423)

git push upstream force-fork-parallel

[forsyth2]

I've confirmed the CI/CD tests pass now. Importantly, that means they're passing for Python 3.11, 3.12, 3.13, and 3.14.

(I couldn't get them to run on #429 for some reason. In any case, that PR merged into a different branch, so I've opened this PR to merge into main and include only relevant, i.e., non-empty, commits).

[forsyth2]

We will merge this for now, to at least have basic support for Python 3.14. The plan is to more fully address the parallelism issues at a later date.