fix(session): switch session when active child/grandchild's ancestor is deleted

[hermandavid]

Problem

When deleting a parent session from the session picker while switched into a child/grandchild subagent session, two bugs occurred:

1. No session switch triggered — the deletion check only compared the active session ID directly against the deleted set, so deleting an ancestor while inside a child session was not detected.

2. Agent mode stuck on subagent — when the "no remaining sessions" fallback created a new session, the agent mode was never reset. If the active session was running under a subagent (e.g. explore), the new session inherited that mode with no way to switch back.

Solution

• Ancestor-aware deletion detection: a new _is_session_or_ancestor_deleted helper walks up the parentID chain to detect whether any ancestor of the active session is being deleted.

• Agent mode reset: when no remaining sessions exist after deletion, state.model.clear() and agent_model.ensure_current_mode() are now called before creating the new session — matching the reset already present in switch_session.

Test coverage

• Unit tests for _is_session_or_ancestor_deleted (direct, parent, grandparent, sibling, unrelated cases)
• Integration tests verifying session switch on ancestor deletion and agent mode reset when all sessions are deleted

[hermandavid]

Failing CI should be unrelated. There was a regression in nightly neovim that was fixed today.

[sudo-tee]

@hermandavid

Thanks for the fix.