Previously, the sidebar would automatically create an empty draft thread
for any project group that had no threads (via `reconcile_groups`). This
added complexity and felt janky — the user would see a phantom "Draft"
entry appear for every workspace they opened, even before they intended
to start a conversation.

Now, project groups can simply be empty. When a group has no threads:

- The fold indicator renders as closed and grayed out (disabled — no
click handler or tooltip)
- The plus button is always visible on the right side of the header (not
hidden behind hover)
- Keyboard fold toggling is disabled for the group
- Clicking the plus button creates the first thread and expands the
group

Draft threads are no longer auto-created anywhere:

- `AgentPanel::load` no longer creates a draft when there is no
previously active conversation
- Archiving the last thread in a group clears the panel instead of
creating a replacement draft
- `show_or_create_empty_draft` and `clear_active_thread` are removed

This removes a significant amount of supporting infrastructure that only
existed to maintain the "no group may be empty" invariant:

- `WorkspaceSidebarDelegate` trait and `AgentPanelSidebarDelegate`
struct
- `reconcile_groups` method and its 12 call sites
- `clear_empty_group_drafts` method and its 3 call sites
- `show_or_create_empty_draft` and `clear_active_thread` methods
- `reconciling` field on `Sidebar`
- `sidebar_delegate` field, setter, and getter on `Workspace`

Release Notes:

- Improved the sidebar to show empty project groups with a visible plus
button instead of auto-creating placeholder draft threads.

---------

Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>