fix: store session archives under PI_WEB_DATA_DIR

[jmfederico]

Summary

• store the session archive index and archived session files under PI_WEB_DATA_DIR, while preserving the existing ~/.pi-web fallback when the variable is unset
• automatically migrate a complete legacy ~/.pi-web archive on the first eligible session-daemon startup, before PiSessionService, route registration, or listening
• require an explicitly configured distinct data root, a valid and self-contained legacy archive, and a pristine destination; ambiguous, invalid, partial, or coexisting state is skipped without archive filesystem mutations
• copy and byte-verify files with exclusive destination creation across filesystem boundaries, rewrite migrated archivePath values while preserving metadata, atomically publish the destination index, and delete legacy archive state only after commit
• stop startup on pre-commit migration failures, continue from a committed destination when legacy cleanup is incomplete, and leave active Pi session files untouched
• document PI_WEB_DATA_DIR as the general root for PI WEB-managed state and keep upgrade-specific migration guidance in the patch Changeset

Verification

• npm run verify — 173 test files passed; 1207 tests passed and 2 skipped
• npm run changelog:status — patch Changeset recognized
• git diff --check
• GitHub CI Verify and package passed on Ubuntu and Windows, including verification, build, and npm package-content checks

Related to #48