Summary

• flatten the complete subagent family in the V2 subagent picker
• render connected tree guides so nested relationships stay visible
• keep the same family visible while navigating at any nesting depth
• add regression coverage for opening the family from a grandchild session

Closes #43082.

Checks

• bun test test/util/session.test.ts in packages/tui
• scoped oxlint (0 errors; 2 pre-existing warnings)
• git diff --check

Full package typecheck could not run cleanly against the host's stale shared workspace dependencies; the changed files produced no typecheck diagnostics after dependency-noise filtering.

Requested by: @neriousy (Filip via Slack)