Install script should use XDG_DATA_HOME for binary location

[thedaneeffect]

Summary

Install script uses ~/.opencode/bin but runtime uses XDG-compliant paths (~/.local/share/opencode/). Should align both to use $XDG_DATA_HOME/opencode/bin.

Changes

• Update install script to use XDG_DATA_HOME (fallback: ~/.local/share)
• Migrate existing installations from ~/.opencode/bin
• Already compatible with upgrade detection (installation/index.ts:60-61)

[github-actions[bot]]

This issue might be a duplicate of existing issues. Please check:

• #5175: can not start opencode on ubuntu /tmp does not allow execution of .so

Feel free to ignore if none of these address your specific case.

[github-actions[bot]]

This issue might be a duplicate of existing issues. Please check:

• #5055: Install doesn't downloads $filename on linux
• #3097: Wrong installation path
• #4170: Reorganize and document dirs for config, agents, auth credentials, sessions

Feel free to ignore if none of these address your specific case.