Cross-Session Context Querying

### Describe the feature or problem you'd like to solve

When working on a complex project, I often have multiple sessions approaching it from different angles. Session A may have already built deep context about the codebase or problem space, but Session B has no way to leverage that understanding — it must re-research everything from scratch or rely on summarized versions of the understanding.

### Proposed solution

Allow a session to semantically query another session's accumulated context — either live (if the target session
is idle) or from its stored checkpoints. This could look like:

1. Cross-session query command — e.g., /ask-session "how does the auth middleware work?"
2. Semantic session store index — vector/embedding-based search over session knowledge, not just keyword FTS5
3. Context export/import — a primitive to serialize a session's synthesized understanding into a portable artifact another session
can ingest cheaply
4. Session-as-agent — treat an idle session as a queryable sub-agent from another session (similar to how /fleet dispatches
background agents, but targeting existing sessions)

### Example prompts or workflows

Use case: Multi-perspective analysis workflows — one session deep-dives into architecture, another focuses on security review, a third
on implementation. Each should be able to tap into the others' findings without redundant exploration, similar to how a team of
engineers would collaborate.

### Additional context

_No response_

この issue にはファイルやテストが指定されていません。まず既存の /fleet コマンドと、セッションのチェックポイントまたはストレージのパスを追跡し、その後、提案されている4つのアプローチのどれを対象とするのかを明確にしてください。完了条件には、ライブまたは保存済みのセッションコンテキストを問い合わせるための定義済みの動作と受け入れテストを含める必要があります。

索引モデルが issue の本文から書いたものです。