feat(memory): expose bounded private memory search and exact retrieval tools

[robinbraemer]

Outcome

Expose bounded private memory_search and exact memory_get behavior so a Mate can deliberately retrieve context without loading an entire memory corpus.

Scope

• Search only the current Mate's validated topics through the selected retrieval interface.
• Return bounded paths, metadata, safe snippets, scores, and degradation state.
• Read exact topics through the existing path-validation and byte-limit contract.
• Respect pause, attachment budgets, topic scope, forget invalidation, and session cancellation.
• Keep automatic pre-turn recall and explicit tools on one ranking/read contract.

Acceptance criteria

• Another Mate's PVC or index is unreachable through either tool.
• Paused or forgotten memory never appears in search or get results.
• Tool results are bounded and do not become telemetry or audit payloads.
• Exact reads revalidate the Markdown source rather than trusting cached chunks.
• The tools remain useful with lexical-only and fallback modes.