Implement memory snapshot export and import

[bug-ops]

Parent: #740 (P2)

Problem

No mechanism for memory portability, backup, or migration between instances.

Solution

• Export: dump SQLite tables + Qdrant points to a portable format (JSON or SQLite file)
• Import: load with conflict resolution (skip, overwrite, merge)
• CLI subcommand: zeph memory export/import

Affected crates

• zeph-memory
• zeph (CLI subcommand)

Acceptance criteria

• Round-trip export/import preserves all data
• Conflict resolution options
• CLI interface