Add OpenClaw session archiving

[thefoxkai]

Summary

• discover every OpenClaw agent's top-level sessions directory
• honor OPENCLAW_STATE_DIR, falling back to ~/.openclaw
• archive all persistent session artifacts while skipping lock and temporary files
• avoid unrelated agent state, including auth data and embedded Codex sessions
• document OpenClaw support and cover it with focused tests

Validation

• uv run pytest
• uv run ruff check .
• uv run ruff format --check .

Live OpenClaw validation against four local agent stores:

• archived 4,542 files totaling 521,624,510 bytes
• compared every source and destination file byte-for-byte: zero missing, extra, or mismatched files
• confirmed all archived files use mode 0600 and directories use mode 0700
• confirmed a stable rerun updates zero files
• confirmed one live transcript change updates one file, followed by another zero-update rerun

[maedmatt]

lgtm