fix(install): guard link-mode conflicts

[fengjikui]

Summary

• Refuse to replace existing non-symlink paths during --link installs unless --force is provided.
• Apply the same conflict check to --dry-run so destructive link-mode conflicts are visible before install.
• Add regression coverage for preserving a user-owned plugins/goal-mode/ directory and for the explicit --force replacement path.

Why

opencode-goal-mode-install --link removed existing destinations before creating symlinks. If a user already had a real plugins/goal-mode/ directory under their OpenCode config, the installer could delete those files without requiring --force. Copy installs already treat conflicts conservatively, so link installs should do the same.

Validation

• Reproduced the original behavior with a temp config containing plugins/goal-mode/keep.txt: install exited 0 and removed the file.
• Verified the fix: --link exits 1 and preserves keep.txt without --force.
• Verified --link --force replaces the directory with a symlink.
• npm test (26 pass)
• npm run lint (passes; existing warnings only in unrelated files)
• git diff --check

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[devinoldenburg]

Nice fix, this is definitely the right direction. One small thing before merging: prepareLinkDestination() uses existsSync(dest), which misses dangling symlinks because it follows the link. With a broken existing symlink, --link --force now skips cleanup and then fails with EEXIST when symlinkSync() runs. Could you switch that check to lstatSync() in a try/catch and treat only ENOENT as missing? A small regression test for a dangling symlink would be great too.