Implement complete workflow for checking and applying OpenCode updates:
- check-version.sh: Compare current vs latest GitHub release
- fetch-sha256.sh: Fetch SHA256 hash using nix-prefetch-url
- update-nix-file.sh: Update Nix package definition with dry-run support
- verify-update.sh: Verify installed version matches expectation
Includes comprehensive documentation (SKILL.md for agents, README.md for users)
and full spec in specs/002-update-opencode/ with user stories and acceptance criteria.
Eliminates manual Nix file editing and hash lookups for OpenCode updates.
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