feat(desktop): support referencing another session from the composer

### Problem

Maka currently supports `@` references for workspace files and structured `QuoteRef` excerpts selected from the current transcript, but there is no user-facing way to reference another session as context.

When useful context already exists in a previous session, users have to manually copy excerpts, repeat the background, or branch from an earlier Turn. Branching is useful for continuing the same history, but it does not cover the case where a new or existing session needs context from a separate session.

### Desired outcome

Allow users to explicitly reference another session from the composer, with a clear chip/preview and deterministic context semantics. For example, the composer could expose sessions through an `@` picker alongside files.

The design should make clear:

- whether the reference captures a snapshot or resolves live;
- how much session content is included and how it fits within the model context window;
- how archived, deleted, or inaccessible sessions behave;
- how provenance is shown to the user and preserved in stored messages.

I am happy to work on this feature if the maintainers agree with the direction.

### Alternatives or workarounds

Today, users can manually copy text into the composer, select individual transcript excerpts as `QuoteRef`s, or branch from a Turn. These approaches either lose session-level provenance, require manual curation, or create a new continuation rather than referencing independent context.

Start by tracing the desktop composer and its existing @ picker, then inspect how QuoteRef, transcript excerpts, Turns, and stored messages are represented. Resolve snapshot versus live semantics, content limits, inaccessible-session behavior, and provenance before implementation. Done means maintainers agree on the design and the resulting reference behavior is specified for these cases.

Written by the indexing model from the issue text.