Add memory_search tool

[theantichris]

Summary

Add a memory_search tool that allows the LLM to search Ghost's persistent memory using hybrid retrieval. Follows the same tool.Tool interface pattern as search.go.

Details

New files:

• internal/tool/memory_search.go
• internal/tool/memory_search_test.go

Type:

type MemorySearch struct {
    Store *memory.Store
}

Tool definition:

• Name: memory_search
• Description: "search your persistent memory for relevant information from past conversations and saved knowledge"
• Parameters:
  • query (string, required): the search query
  • max_results (integer, optional): maximum results to return (default 5)

Execute:

• Parse args JSON (same pattern as search.go)
• Call Store.Search(ctx, query, maxResults)
• Format results:

  Memory Result 1: [source: daily/2026-02-07.md]
  <chunk content>
  
  Memory Result 2: [source: MEMORY.md]
  <chunk content>
  

Errors:

• ErrMemorySearchArgs — failed to parse arguments
• ErrMemorySearchFail — search operation failed

Testing:

• Table-driven tests following search_test.go pattern
• Use real memory.Store with temp dir and mock EmbedFunc

Note: This tool is not registered in the registry yet — that happens in the integration issue.

Dependencies

• Add memory storage engine #236 (memory storage engine)

Acceptance Criteria

• Implements tool.Tool interface (Definition() + Execute())
• Returns formatted search results with source attribution
• Defaults to 5 results when max_results not specified
• Handles empty results gracefully
• Proper error handling for invalid args and search failures
• Table-driven tests pass