fix(runtime): deliver the history-compact instruction as a trailing user message

[ggbdpq]

Summary

Fixes the root cause of #4634: the history summarizer delivered SUMMARIZATION_SYSTEM_PROMPT as provider instructions (system role). Agentic coding models such as kimi-coding-plan / k3-256k ignore that shape and keep answering the conversation, so every fold failed open with malformed_summary_missing_section, and the distinct system prompt also forfeited the main loop's prefix cache for every discarded summarizer call.

The instruction now rides as the request's trailing user message — the request already ended with one for chat-template models (#4559); generateSummary appends a single user message combining the sectioned prompt (or its shorten/repair variant) with the existing summary request, and AiSdkGenerateTextOptions drops the now-unused instructions field.

Scope: direction 1 from the issue. Of the contributing defects listed there, the missing output cap is already fixed on current main by #4653 (DEFAULT_HISTORY_COMPACT_MAX_OUTPUT_TOKENS, landed 09-03, after the reporter's build); extending the failure circuit to provider_error/output_length and bounding the summarizer input budget remain open follow-ups best split into their own issues.

Verification

Claim	Command	Result
Targeted suites green	npm --workspace @maka/core run build && npm --workspace @maka/storage run build && npm --workspace @maka/runtime run build, then node --test on history-compact-summarizer / effective-history-compaction / mid-turn-capacity-backend dist tests	139 tests, 139 pass, 0 fail
Red-green: the new test fails without the fix	git stash push -- packages/runtime/src/history-compact-summarizer.ts, rebuild, rerun the summarizer suite	53 tests, 3 fail (the new trailing-instruction test + the two variant-prompt captures); restored → 53/53 pass
New regression test	delivers the summarization instruction as the trailing user message (#4634)	asserts no instructions field reaches generateText and the final user message carries both the sectioned prompt and the summary request
Format	npm run format:check	exit 0, 1,942 files clean
License headers	npm run check:asf-headers	both changed files pass

AI use

Implemented with ZCode (GLM-5.3-Flash): read the issue's reproduction data, traced the summarizer request shape in packages/runtime/src/history-compact-summarizer.ts, and made the minimal request-shape change with a red-green regression test. The commit carries the Generated-by trailer.

Checklist

• Root-cause fix in the shared summarizer builder; no per-provider branching requested by the issue
• Regression test fails on the old code (verified via stash/rebuild)
• Test style follows the existing file (node:test + assert/strict, issue number in the test name)
• No new dependencies; docs unchanged