Add batch endpoint for memories (HTTP + MCP + CLI) #53

Summary

Adding multiple memories currently requires one HTTP call / MCP tool call per
memory (N subprocess spawns for N memories). This is slow and rate-limit-prone.
Add a batch add endpoint so N memories can be inserted in a single call.

Proposal

• HTTP: POST /api/memories/batch
  body: { "memories": [ {"content": "...", "metadata": {...}}, ... ] }
  response: { "result": { "status": "success", "added": N, "memory_ids": [...] } }
• MCP: add_memory accepts either content (string, single) OR memories
  (array, batch) — backward compatible.
• CLI: memayu add --batch file.jsonl (or pipe jsonl via stdin)

Acceptance criteria

• Single call inserts N memories with one embedding round-trip.
• MCP add_memory supports array argument.
• CLI --batch supports jsonl file + stdin.
• Errors are per-item (one bad memory doesn't fail the whole batch).