…revent stale instructions execution

Addresses issue where stale '## Pending User Asks' and '## Remaining Work'
sections in compression summaries were incorrectly executed as current tasks
when they had topical overlap with the latest user message.

Changes:
1. STRENGTHENED PREAMBLE: Rewrote SUMMARY_PREFIX with explicit 'STALE — DISCARD'
   language, replacing weak 'treat as background reference' with:
   - ⚠️ CRITICAL RULE section explicitly listing dangerous section names
   - Bullet points identifying stale items (Pending Asks, Remaining Work,
     In Progress, Blocked, Completed Actions)
   - Any phrasing that sounds like 'next steps' or 'to-do'

2. NEGATIVE EXAMPLES: Added three concrete before/after examples showing:
   - Wrong approach (acting on stale summary items)
   - Correct approach (discarding stale items, responding to latest message)
   - Covers auth, API endpoint, and multi-priority scenarios

3. CLARITY ON MEMORY vs SUMMARY: Explicitly contrasted:
   - Memory (MEMORY.md, USER.md) = eternal, authoritative
   - Summary sections = transient, must be discarded

4. RENAMED DANGEROUS SECTIONS in fallback template:
   - '## Pending User Asks' → '## Protected Context (Background Reference Only)'
   - '## Remaining Work' → '## Session Context (fallback only)'

5. UPDATED LLM SUMMARIZER TEMPLATE to prevent creation of dangerous sections:
   - Replaced '## Pending User Asks' section with '## Protected Context'
   - Removed '## Remaining Work' (fold into Critical Context)
   - Added explicit instruction: 'Do NOT phrase as pending or remaining —
     phrase as completed background facts'
   - Added critical warning: 'Do not create a ## Pending User Asks section
     or anything that looks like unfinished work — the model will execute it'

Root cause: The previous preamble's soft language ('treat as background reference')
was insufficient when summary items had topical overlap with the latest message.
Models would conflate stale summary items with current context.

This fix makes the discard rule explicit, unavoidable, and surrounded by
concrete examples so the intent is unambiguous.

Fixes NousResearch#41607