Add [memory.compression.probe] configuration option to the interactive --init wizard (config migration).

Related to: PR #2047 (compaction probe validation #1609)
Phase: 3 (post-MVP integration)

Users should be able to configure probe settings via --init:

• probe_enabled (default: false)
• probe_model (default: inherits summary_model)
• probe_threshold (default: 0.6)
• hard_fail_threshold (default: 0.35)

Also handle: Config migration for existing zeph.toml files to inject [memory.compression.probe] section with defaults.