docs(spec): unify the helper section after the day's five revisions

The detection and identity subsections were authored under one design and patched under two others. They're now three pieces: the roster as the single detection primitive, two spawn paths (deterministic launcher, startup safety net) sharing the same three steps, and identity plus the role contract, with helper-mode.org named the single home of the helper rules.

Verified ai-term.el's launch mechanics in code: the startup instruction is embedded in the tmux new-session command string, so env injection is a prefix on that string, and the -A flag means one session per project, so the helper path needs its own session and buffer names. Added ai-term recommendations to ride the same handoff (roster-derived badges, agent-exit visibility, one source for opener strings) and reconciled the v0 session-context symlink question with the Phase 1.5 answer.

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