feat(session-convert): write derived sessions to native directory by default

[SurviveM]

Requirement

Change buildDerivedSessionOutputDir default behavior: write converted sessions to target tool's native session directory (Codex: ~/.codex/sessions/; Claude: ~/.claude/projects/<project>/) instead of ~/.codexmate/sessions/derived/. Add --output-dir derived flag to preserve current behavior. Check sessionId conflicts before writing.

Verification

1. codexmate convert-session --from codex --to claude --session-id xxx -> file appears directly in ~/.claude/projects/ -> claude -r <derived-id> works without import step
2. --output-dir derived -> behavior reverts to current (writes to ~/.codexmate/)
3. Convert with duplicate sessionId -> conflict warning displayed, conversion aborted until resolved