chore: update plugin ref to opencode-conventional-commits@0.0.1

refactor: rename ParseError to CommitError, tidy Result and validator

refactor: move ParseError to errors.ts, remove try/catch from tests

refactor: replace errors.ts with ParseError class, use instanceof

refactor: rename to opencode-conventional-commits, flatten utils

feat: add git-undo tool, git-diff staged param, update README and instructions

test: add config and tools tests, document unsupported features

chore: bump version to 0.0.1-next.0

feat: initial opencode-commits plugin