feat: project-local state default with env override and XDG migration

Resolve the state-file path as stateFilePath option > OPENCODE_GOAL_STATE_PATH
env var > project-local <cwd>/.opencode/goals/state.json (previously the
home-dir default). When the default path is empty, migrate forward on first
load from the legacy ~/.opencode-goal-plugin path and the XDG state path,
then persist to the project-local path. Explicit option/env paths are
literal with no fallback, and a corrupt primary file is preserved.

Refactors the loader into applyParsedStateFile + a candidate loop, adds
resolveStateFilePath/xdgStateFilePath/legacyStateFilePaths, README/CHANGELOG,
and unit + migration tests. Items 6.1 and 6.2.