fix(agentmain): make /resume fall back to L4 archives (closes #630)

[Kailigithub]

Problem

Issue #630: /resume stops listing historical sessions after L4 archival
(memory/L4_raw_sessions/compress_session.py) has run.

The /resume handler in agentmain.py hardcoded the prompt to read
temp/model_responses/, but the archival pipeline deletes the raw .txt
files once it has merged them into all_histories.txt and the monthly
{YYYY-MM}.zip archives. The data is still recoverable — the agent just
isn't told to look for it.

Fix

Single-source change in agentmain.py: extend the /resume prompt so the
agent falls back to the L4 archives when fewer than 5 files remain in
temp/model_responses/.

if raw_query.strip() == '/resume':
    return (r'帮我看看最近有哪些会话可以恢复。'
            r'先列 temp/model_responses/ 目录（按修改时间取最近10个文件）。'
            r'如果该目录里的 .txt 少于 5 个（L4 归档已清理原始文件），'
            r'回退读 memory/L4_raw_sessions/all_histories.txt（汇总的会话历史），'
            r'或解压 memory/L4_raw_sessions/{YYYY-MM}.zip 取最近的月度归档。'
            r'从每个文件里找最后一个<history>...</history>块，'
            r'用一句话总结每个会话在聊什么，列表给我选。'
            r'注意读文件后要把字面的\n替换成真换行才能正确匹配。')

Verification

/tmp/test_issue_630.py checks (a) the old prompt string is gone,
(b) the new prompt mentions all_histories.txt, memory/L4_raw_sessions,
{YYYY-MM}.zip, and the fallback threshold, and (c) the /resume branch
is still wired. Three-step dance (stash → test fails → pop → test passes)
confirms the test catches the regression.

python3 -m py_compile agentmain.py     # syntax
python3 /tmp/test_issue_630.py          # behaviour

Scope

• 1 file, +8/-1 lines
• Pure prompt string change — no runtime logic, no schema impact, no migration
• Closes /resume 在 L4 会话归档后失效，无法感知已归档的历史会话 #630