Add memory_save tool

[theantichris]

Summary

Add a memory_save tool that allows the LLM to persist important information to Ghost's memory system. Follows the same tool.Tool interface pattern as search.go.

Details

New files:

• internal/tool/memory_save.go
• internal/tool/memory_save_test.go

Type:

type MemorySave struct {
    Store *memory.Store
}

Tool definition:

• Name: memory_save
• Description: "save important information to persistent memory for future reference"
• Parameters:
  • content (string, required): the information to save
  • target (string, optional): "daily" (default) or "long_term"

Execute:

• Parse args JSON (same pattern as search.go)
• Route to Store.SaveToDaily() or Store.SaveToLongTerm() based on target
• Return confirmation: "Saved to daily log (2026-02-07.md)" or "Saved to long-term memory (MEMORY.md)"

Errors:

• ErrMemorySaveArgs — failed to parse arguments
• ErrMemorySaveFail — save operation failed
• ErrInvalidTarget — target not "daily" or "long_term"

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())
• Saves to daily log by default
• Saves to MEMORY.md when target is "long_term"
• Returns clear confirmation message
• Proper error handling for invalid args and save failures
• Table-driven tests pass