[feat] Hint the user to start a new session after N conversation turns

[jeff-r2026]

Problem

Long-running AI coding sessions accumulate large context windows, which degrades response quality and increases cost. There is currently no signal nudging the user to start a fresh session once a conversation has gone on for many turns.

Proposed Solution

Add a new prompt-submit (UserPromptSubmit) hook handler that counts human conversation turns per session and injects a gentle, one-time reminder when the count crosses a threshold.

• Threshold: default 20 turns, overridable via TEAMAI_TURN_LIMIT.
• Frequency: hint once per session (fires on the first crossing, then stays quiet).
• Kill switch: TEAMAI_TURN_HINT_DISABLED=1, independent from TEAMAI_RECALL_DISABLED.

Why this fits cleanly

• The hook dispatcher already fires on prompt-submit for every user message (trackSlashHandler is already wired there), so one user turn == one prompt-submit.
• Handler return values are injected back into the session by the host (hookSpecificOutput.additionalContext).
• Per-session counting + 24h TTL dedup is an established pattern (todowrite-hint.ts).

Net change: one new handler + one small module. No dispatcher changes, no new command.

Implementation sketch

1. src/turn-limit-hint.ts — count store at ~/.teamai/sessions/<sid>-turn-count.json (24h TTL), resolveTurnLimit(), recordTurnAndShouldHint(), buildTurnLimitHintMessage().
2. Register turnLimitHintHandler on prompt-submit in hook-handlers.ts.
3. Unit tests (threshold, once-only, TTL reset, env override, disable switch).
4. End-to-end verification with the real CLI via hook-dispatch prompt-submit.
5. Sync docs (README + README.zh-CN + usage-guide both languages).

Alternatives Considered

• Count on stop instead of prompt-submit: rejected — stop fires per assistant turn including tool loops, so it doesn't map to "human turns".
• Hard-block via continue:false: rejected — too disruptive; a soft additionalContext nudge lets the model relay it at a natural moment.
• Reuse TEAMAI_RECALL_DISABLED as the switch: rejected — coupling unrelated features; a dedicated switch is clearer.

Additional Context

Turn semantics: counts UserPromptSubmit events only; subagent round-trips and tool calls do not count, matching the "human conversation" intent.

[lurkacai0831]

已提交 PR #257 实现该功能。

主要实现：

• 在 prompt-submit 上计数用户轮次
• 在 stop 上输出可见提醒
• 默认阈值 20，支持 TEAMAI_TURN_LIMIT 覆盖
• 支持 TEAMAI_TURN_HINT_DISABLED=1 永久关闭
• 支持回复 "关闭轮次提醒" / "disable turn hint" 静默当前会话