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_