* fix(pi): add get_session_stats fallback from get_state for old OMP binaries
Older Oh My Pi binaries don't support the get_session_stats RPC command,
leaving session usage stats blank in Paseo. This adds a compat layer in
cli-runtime.ts where getSessionStats() tries get_session_stats first, then
falls back to extracting context window usage from get_state.
The fallback provides contextUsage.tokens and contextWindow even when the
binary lacks full token/cost reporting — enough to keep the context meter
working. Token counts and cost still require the newer RPC.
Added three tests:
- Fallback triggers when get_session_stats is unsupported (throws)
- No fallback when get_session_stats returns valid data
- Returns empty object when both commands fail
* fix(pi): use nullish check instead of truthy for cost field in getSessionStats
