Add context management documentation guide

[kaleko]

Adds docs/CONTEXT_MANAGEMENT.md covering:

• Sliding window (Strands built-in)
• Summarizing conversation manager (Strands built-in)
• LangGraph middleware (trim/summarize)
• Custom hook-based context management with external memory re-injection

Includes strategy comparison, code examples for each approach, best practices, and guidance on applying to existing FAST agent patterns.