Project pane should show subfolders of an opened parent directory as a tree

When you open a parent directory as a session, only that directory appears in the left pane. Its subfolders — each a distinct project — are invisible until you manually start a separate session in each one.

This breaks the mental model the pane's name suggests. "Projects pane" reads as a view of your projects; it's actually a list of sessions, one entry per session cwd, with no relationship between a parent directory and the projects inside it.

Concrete case: a development directory with 10 project subfolders. Opening it yields exactly one pane entry. There is no indication the other 10 exist, no way to add them short of starting 10 separate sessions, and nothing in the UI explains why they're absent. The natural conclusion when projects vanish from the pane is that they were archived — leading to hunting through an Archived list that doesn't contain them, because they were never there.

It also has a real functional cost. Per-project CLAUDE.md and AGENTS.md files load only when a session's cwd is that project folder. A session opened at the parent silently loads none of them. So the natural "open my dev folder" workflow is the one that quietly drops every project's instructions.

Requested behavior: when a directory containing multiple project folders is opened, show those subfolders as a tree under the parent in the pane, with each expandable into its own session that inherits the correct cwd and picks up that folder's instruction files. Detecting child projects by the presence of .git, CLAUDE.md, AGENTS.md, or package.json would cover most cases.

Smaller alternative, if the tree is too large a change: when a session opens on a directory whose subfolders look like projects, surface a hint — "10 project folders found here; open sessions for them?" That alone would prevent the "my projects disappeared" confusion.