Add OMP session archiving

[thefoxkai]

Summary

• archive Oh My Pi (OMP) session artifacts and referenced media from $PI_CODING_AGENT_DIR or ~/.omp/agent
• preserve transcripts and their associated subagent summaries and tool logs
• skip lock and temporary files while leaving OMP databases, caches, binaries, and credentials outside the archive

Validation

• uv run ruff check .
• uv run ruff format --check .
• uv run pytest (13 passed)
• live archive of the local OMP store: 355 files / 90,183,209 bytes
• every archived file compared byte-for-byte: 0 missing, extra, or mismatched
• stable rerun copied 0 files
• archive permissions remained 0700 for directories and 0600 for files

[maedmatt]

working hard xD