v0.3.0 — per-session phase honesty

Drops the public GLOBAL_SESSION_ID constant and the migrated-sentinel
fallback in check_phase. Per-session phase is now honest at the
workflow layer; callers must thread an explicit session id.

Recovery from migrated v1 estates still works via:
  synthesist phase show --session=global-migrated

Breaking: pub const removal and check_phase behavior change on None
session.