[Process] Update AGENTS_STARTUP.md to mandate reading recent session summaries

Based on recent analysis, reading recent session summaries provides critical "institutional memory," preventing regression and aligning strategic context.

Requirement: Update AGENTS_STARTUP.md (specifically Section 6: Check for Memory Core) to:

1. Mandate that agents execute get_all_summaries({ limit: 5 }) if the Memory Core healthcheck is successful.
2. Explain the value: Explicitly state that this establishes continuity and helps identify recent failures or architectural decisions.
3. Encourage Drill-Down: Add a note that agents can and should use get_session_memories to investigate specific sessions if a summary indicates relevant details (e.g., a "failed" session or complex refactor).

Success Criteria:

• AGENTS_STARTUP.md clearly instructs agents to perform this action during initialization.
• The instruction is framed as a mandatory step for "Memory Core Active" scenarios.