feat(bstack): add logs, errors, leaks, session commands + Doppler auto-load

New investigation commands:
- bstack logs <user|keyword> — search by user/keyword with --level, --duration,
  --region, --service, --env filters. Auto-selects hot vs historical table.
- bstack errors --region <r> — top errors by count (replaces raw SQL)
- bstack leaks --region <r> — memory leak detection (disposedSessionsPendingGC
  trend, GC freed amounts, MemoryLeakDetector events, auto-diagnosis)
- bstack session <userId> --host <h> — live session inspection via admin API

Infrastructure improvements:
- Doppler auto-load: credentials auto-loaded from mentra-sre project if env
  vars not set. No more wrapping every command in 'doppler run'.
- Auto table selection: getTableForDuration() picks hot storage (<5min) vs
  historical/S3 (>5min) based on --duration flag. Users never need to know
  about table topology.
- Help text: organized into Investigation / Diagnostics / Infrastructure
  sections, cross-references runbooks, shows credential sources.

All tested: logs, leaks, health, help confirmed working.

Author: isaiahb