fix(scroll): anchor the live turn with a seam banner in the eviction index

[niceIrene]

Description

After scroll compresses old turns, the context looks like:

System(prompt)
User  → eviction index (a map of archived turns, each with a ⟦headline⟧)
User  → the real current request
Assistant …

The index and the request are both user messages (the index can't be a system message — Anthropic needs the first body message to be user). Since the index sits right before the request and lists old ⟦headlines⟧, weaker models (GLM, DeepSeek) sometimes answer an old headline instead of the current request (e.g. ask "what is GitHub", agent starts answering the earlier "what time is it").

Fix: clearly label where the archive ends and the live turn begins.

┌─ System ──────────────────────────────────────────────────────
│  <system prompt…>
│
├─ User → eviction index (archived map) ────────────────────────
│  <system-info>
│  [context compressed] This is an ARCHIVED MAP for reference only
│  — NOT the live conversation. Read it top (oldest) to bottom.
│  Each '·' line is a seq span you can re-expand with recall_history.
│
│  ===== Tier 0 (recently compressed) =====
│    [seq 1–2]
│      · seq 2  ⟦ user asked what time it is ⟧
│    [seq 3–5]
│      · seq 5  ⟦ answered: it's 14:20 ⟧
│
│  ═══════════ END OF ARCHIVED INDEX ═══════════        ← seam banner (new)
│  The CURRENT LIVE TURN is the message(s) below. Answer the most
│  recent USER message there — NEVER a ⟦headline⟧ in the map above;
│  those are archived, not requests.
│  </system-info>
│
├─ User → the real current request ─────────────────────────────
│  What is GitHub?                            ← sits right under the banner
│
└─ Assistant ───────────────────────────────────────────────────
   <starts explaining GitHub…>

Now the "answer the message below, not a headline above" signal is the last line before the real request — right where the confusion happens. Before, that guard was buried at the top of the index. Same idea as hermes-agent's end-of-summary marker. describe() (the /compact reply shown to users) is unchanged — the banner is model-only, and it's constant text so KV-cache is unaffected.

Related Issue: N/A

Security Considerations: N/A

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation
• Refactoring

Component(s) Affected

• Core / Backend (app, agents, config, providers, utils, local_models)
• Console (frontend web UI)
• Channels (DingTalk, Lark, QQ, Discord, iMessage, etc.)
• Skills
• CLI
• Documentation (website)
• Tests
• CI/CD
• Scripts / Deploy

Checklist

• I ran pre-commit run --all-files locally and it passes
• If pre-commit auto-fixed files, I committed those changes and reran checks
• I ran tests locally (pytest or as relevant) and they pass
• Documentation updated (if needed)
• Ready for review

Testing

Two new cases in tests/unit/agents/context/test_eviction_index.py:

• test_render_closes_with_live_turn_banner — banner appears after the archived turns, inside </system-info>.
• test_describe_omits_the_model_facing_banner — the /compact reply stays banner-free.

Local Verification Evidence

python -m flake8 src/qwenpaw/agents/context/scroll/eviction_index.py tests/unit/agents/context/test_eviction_index.py
# clean, exit 0

python -m pytest tests/unit/agents/context/ tests/unit/runtime/test_builtin_commands_scroll.py tests/unit/runtime/test_scroll_recall_gate.py -q
# 181 passed

[github-actions]

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