Add create_thread and list_agents_and_models agent tools

[eholk]

Adds two built-in agent tools, behind the staff-only create-thread-tool feature flag, that let the agent spawn independent sibling threads:

• create_thread creates a new thread with a title and initial prompt. It runs on the user's behalf and shows up in the agent sidebar like a thread the user started themselves, but it runs in the background — the active thread is never interrupted — and it does not report results back to the caller. (Use spawn_agent when you want the results.)
• list_agents_and_models enumerates the configured agents and, for the native agent, the available models, so the agent can pick a cheaper/faster model for bulk work. The motivating stress test was "spawn 100 threads that each write a haiku, using Haiku instead of Opus."

By default the sibling shares the parent's project and worktree. If use_new_worktree is set, create_thread instead opens a new background workspace backed by fresh linked git worktrees of the project's repos — the same flow you get from the worktree picker's "Create worktree" — and starts the thread there. The new workspace opens as a background tab so it doesn't pull you away from what you're doing.

A few notes on the design:

• The agent crate can't depend on the UI, so there's a SiblingThreadHost trait that agent_ui::AgentPanel implements and installs on the NativeAgent. Tool calls in a native-agent thread route through it.
• Sibling threads go through the same create_agent_thread → ConversationView path as the user-facing "new thread" action, so eviction, persistence, and sidebar rendering match user-created threads.
• An explicit agent/model override doesn't clobber the panel's selected agent or the last-used-agent preference — those are snapshotted and restored around the call.
• The worktree path reuses the existing "Create worktree" machinery rather than reimplementing it; the agent flow just opens the new workspace in the background instead of activating it.

A couple of things I left out or want to call out:

• There's no tool to operate on an already-created sibling by session id. The success payload deliberately omits it, since the session is populated asynchronously and nothing consumes it yet.
• When a project has multiple worktrees that are linked worktrees of the same underlying repo, they collapse into a single new worktree (they resolve to the same target path). The tool returns a warning in that case rather than failing — and this also fixes a latent collision the manual "Create worktree" flow would hit in the same situation.

Release Notes:

• N/A