Session-naming tip: teach once at join, remind once at first unnamed actas (run/ sentinel), then stay silent

[fujibee]

Problem

The /rename <team>-<agent> tip (#339/#344) currently fires on EVERY actas of a non-spawned session. Two annoyances: users who deliberately don't rename get nagged every session, and manually resumed sessions get re-tipped even though they already carry the conventional name (env gate doesn't survive resume).

Design (approved)

Split the tip into education + a one-shot reminder, both governed by a single shared sentinel:

1. Join time (education, once): the join flow explains the naming convention — sessions named <team>-<agent> are findable in the resume picker and restorable after a restart. Shown as part of first-time setup output.
2. Actas time (reminder, once): the existing tip stays where it is (non-spawned sessions only, AGMSG_SPAWNED gate unchanged) but fires only if the sentinel is absent.
3. Shared sentinel: run/tip.rename.<team>__<agent> (same sanitized naming scheme as the actas locks). Written best-effort when EITHER surface shows the message; either surface suppresses when present. Sentinel is advisory runtime state — safe to delete, recreated on next show.
4. When the opt-in pane-title feature (Universalize the <team>-<agent> seat marker: set the tmux pane title at every actas (opt-out via env) #369) is active for the session, the actas reminder is suppressed too (the seat marker need is already met).

Notes

• One sentinel per (team, agent), not per session — the goal is 'the human has been told about this role once', not per-session tracking.
• Fits the no-nagging / minimal-intervention direction: knowledge once, nudge once, silence after.