fix(tui): keep the subagent activity filter across navigation - #53318

What

ctrl+a in the subagents composer is now a pure toggle: choosing a subagent with enter keeps the active/inactive filter instead of snapping back to "active".

Why

The session route is keyed by sessionID, so navigating to a subagent remounts the composer and SubagentsTab recreated its store with active: true. Closing the composer also explicitly reset the filter.

How

• Keep the filter in useStorage().memory("subagents-filter") so it survives the route remount for the life of the TUI process (not persisted to disk).
• Only composer.subagent.toggle-activity changes it; closing the composer still resets the selection, but no longer the filter.

Verification

• New test in composer-keymap.test.tsx mounts the composer keyed by route session like the app does, toggles to inactive, presses enter, and checks that the child session still shows inactive subagents, then toggles back. It fails on origin/v2 and passes with this change.
• bun test test/cli/tui/composer-keymap.test.tsx (7 pass) and bun run typecheck in packages/tui.
• End-to-end opencode-drive run against origin/v2 (4fb8007) and this branch, with one finished and one running background subagent: before, enter on the finished subagent reopens the picker on "Running" only. After, the picker stays on inactive and ctrl+a still flips it back. The demo video is in the requesting Slack thread, because the GitHub App token can't upload PR attachments.

Requested by: @nexxeln (nexxel via Slack)