docs: Improve working memory documentation

[abrookins]

Restructures the working memory documentation to be clearer for newcomers and properly document automatic summarization.

Changes

Restructured for clarity:

• New opening that immediately explains what working memory is
• "What Working Memory Does" bullet list upfront

New section: Automatic Summarization

• Explains what happens when conversations get too long
• Visual diagram showing how messages become context
• Where to find the summary (context field)
• How to monitor summarization progress (context_percentage_* fields)
• Configuration options (SUMMARIZATION_THRESHOLD, GENERATION_MODEL, PROGRESSIVE_SUMMARIZATION_PROMPT)

Removed/consolidated:

• Removed redundant "Automatic Promotion" section (was duplicated)
• Removed verbose "Memory Lifecycle" section (covered elsewhere)
• Removed "Best Practices" section (points already made inline)
• Streamlined TTL section