fix: back up stow conflicts before make install

[fullstackjam]

What does this PR do?

Pre-backs-up existing regular files that would conflict with stow before running make install, mirroring the logic already in linkWithStow.

Why?

The v0.61.0 release workflow failed at the vm-e2e stage:

stow -v --target="/Users/runner" git ssh zsh
WARNING! stowing git would cause conflicts:
  * cannot stow .dotfiles/git/.gitconfig over existing target .gitconfig since neither a link nor a directory
WARNING! stowing zsh would cause conflicts:
  * cannot stow .dotfiles/zsh/.zshrc over existing target .zshrc since neither a link nor a directory
All operations aborted.
make: *** [install] Error 1
✗ Dotfiles setup failed: link dotfiles: make install: exit status 2

PR #123 added linkWithMake to prefer a Makefile's install target over calling stow directly, but didn't carry over the backupConflicts step that linkWithStow has always run. The default dotfiles repo's Makefile delegates to stow, which aborts on pre-existing files it can't replace. On the vm-e2e runner, .gitconfig and .zshrc exist from earlier installer steps.

Testing

• go vet ./... passes
• Relevant tests added or updated — two new unit tests: TestLinkWithMake_BacksUpConflictsBeforeRunning and TestLinkWithMake_RestoresConflictsOnMakeFailure
• Tested locally (./openboot install --dry-run or similar)

Cross-repo checklist

• Does this need a docs/content update in openboot.dev? — No
• Does this change the CLI ↔ server API contract? — No

Notes for reviewer

linkWithMake now:

1. Scans each non-hidden subdirectory (stow package) for regular-file conflicts
2. Backs them up to .openboot.bak before running make install
3. Restores them if make fails
4. Cleans up backups on success

This is the same pattern linkWithStow has used since the feature landed.