pi: write the error log without blocking the event loop #90

The pi integration's default onError is writeErrorLog, which runs statSync, a possible renameSync, and appendFileSync on pi's thread before it returns. A slow agent directory, such as a network mount set through PI_CODING_AGENT_DIR, holds up pi's event loop and TUI for as long as each write takes.

right now:

• each onError call does the stat, rotation, and append synchronously; errors are caught, but the wait is not
• an export outage reports once after fix(sdk): report export outages once and keep errors off agent TUIs #86, but other errors still report per call, for example a mask hook that throws on every span

expected:

• writes and rotation happen off the calling path, so onError returns without waiting on the disk
• pending writes are bounded, entries past the bound are dropped, and a failed write never rejects into pi
• the log file keeps mode 0600, rotates past 5 MB, and holds entries in call order
• the existing pi test still finds the entry in telemetry-dev.log after the session flushes

relevant files:

• packages/pi/src/error-log.ts
• packages/pi/src/extension.ts
• packages/pi/tests/pi.test.ts