Add OpenCode source parsing support

[KNN-07]

Summary

• add opencode as a first-class source in parser discovery and session parsing
• parse OpenCode SQLite session/message/part records into DataClaw's normalized export shape, including model and token stats
• update CLI source options/messages and add tests for OpenCode discovery/parsing and source-choice handling

Verification

• PYTHONPATH=. pytest (245 passed)

[peteromallet]

Thanks @KNN-07! Merged via #10 which rebases your work onto current main (after Gemini and subagent PRs landed) and resolves all conflicts. You're credited as co-author on the commit. Nice clean implementation — the SQLite parsing and OpenCode schema handling are well done.