Scope prompt recent history by session

[chengyongru]

Summary

• Add optional session_key metadata to history.jsonl writes from summaries and raw archive fallbacks.
• Filter # Recent History prompt injection by current session for non-unified mode.
• In unified mode, keep user-visible/global history while excluding internal cron/dream/heartbeat sessions; cron sessions can still see their own cron history.

Verification

• $verify: external package-consumer script after python -m pip install -e . passed non-unified, unified-user, and unified-cron prompt checks.
• python -m pytest tests/agent/test_memory_store.py tests/agent/test_context_prompt_cache.py tests/agent/test_consolidator.py tests/agent/test_unified_session.py tests/cron/test_cron_tool_list.py tests/cron/test_cron_service.py -q — 192 passed.
• ruff check nanobot/agent/memory.py nanobot/agent/context.py nanobot/agent/loop.py tests/agent/test_memory_store.py tests/agent/test_context_prompt_cache.py tests/agent/test_consolidator.py — all checks passed.
• $simplify: reduced unified visibility filtering and raw archive callback boilerplate; focused checks still passed.

Fixes #4259