feat: improve long-term memory docs and discoverability

[Naomi Pentrel (npentrel)]

Description

Improves the long-term memory documentation and its discoverability from the short-term memory page.

The long-term memory page was missing setup guidance — it jumped straight into the store API without showing how to connect a store to an agent. Users on the short-term memory page also had no way to discover that long-term memory exists, since it's in a different navigation section ("Advanced usage" vs "Core components").

Changes:

• Short-term memory page: Added a <Tip> callout linking to long-term memory docs, placed after the overview section
• Long-term memory page:
  • Added SEO description frontmatter
  • Rewrote overview to be clearer and link back to short-term memory
  • Added a new "Usage" section showing how to pass a store to create_agent (Python + JS)
  • Added "In production" subsection with PostgresStore examples (Python + JS), matching the pattern used on the short-term memory page
  • Added a <Tip> linking to the Memory conceptual guide for deeper reading
  • Added a "Related" section at the bottom with cross-links to short-term memory, conceptual guide, and persistence docs

Resolves DOC-800

Test Plan

• Verify the short-term memory page shows the Tip callout linking to long-term memory
• Verify the long-term memory page renders correctly with the new Usage and In production sections
• Verify all internal links resolve correctly
• Verify code examples use correct imports and API patterns (matched against existing examples in the codebase)

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Mintlify preview ID generated: preview-opensw-1772802747-8eae154

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Mintlify preview ID generated: preview-opensw-1772813421-69b83ed

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