Adds a new `gt doctor` check that verifies tmux session environment
variables match expected values from `config.RoleEnvVars()`.
- Checks all Gas Town sessions (gt-*, hq-*)
- Compares actual tmux env vars against expected for each role
- Reports mismatches with guidance to restart sessions
- Treats no sessions as success (valid when Gas Town is down)
- Skips deacon (doesn't use standard env vars)
Also:
- Adds `tmux.GetAllEnvironment()` to retrieve all session env vars
- Removes redundant gtroot_check (env-vars check covers GT_ROOT)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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