[repo-assist] fix: honor --redownload for explicit AUR targets

[github-actions]

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Closes #2973

Root cause

The new workspace preparer only treated redownload=all as an unconditional refresh. The redownload=yes value fell through to the cached .SRCINFO version check, so it behaved like noredownload.

Fix

• Force a PKGBUILD refresh when redownload=yes is used for an explicit AUR target.
• Preserve the existing version check for AUR dependencies.
• Preserve unconditional refresh behavior for redownload=all.
• Add regression coverage for all three modes.

Trade-offs

The implementation uses the existing InstallInfo.Reason distinction between explicit targets and dependencies, avoiding changes to workspace preparation APIs or download behavior.

Test Status

• go test ./pkg/sync/workdir — passed
• make build — passed
• git diff --check — passed

No new dependencies were added.

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