Add create_thread and list_agents_and_models agent tools

[eholk]

Adds two new built-in agent tools that let the agent spawn independent
sibling threads which appear in the agent sidebar and run in parallel with
the current conversation:

• create_thread — creates a new thread with a user-supplied title and
  initial prompt. The new thread runs on the user's behalf, persists like any
  other thread, and does not report results back to the parent (use
  spawn_agent for that). The parent thread's active view is never
  interrupted.
• list_agents_and_models — lets the agent enumerate configured agents
  and, for the native agent, the models that are available. Useful for
  choosing a cheaper model for bulk work (the motivating stress test:
  "spawn 100 threads that each write a haiku, using Haiku rather than Opus").

Behavior

• The new thread shares the parent's project and worktree.
• It's inserted into AgentPanel.retained_threads just like a user-created
  thread, subject to the same MaxIdleRetainedThreads eviction and
  ThreadMetadataStore persistence. Running threads are never evicted;
  finished threads are rehydrated from disk on click.
• create_thread supports optional agent and model overrides. An agent
  override does not mutate the panel's selected_agent or the
  last-used-agent preference — those are snapshotted and restored around the
  call.
• A use_new_worktree field is on the schema but currently returns an error;
  the flag is there so follow-up work can wire it through to the existing
  handle_worktree_requested flow without a schema change.

Architecture

• agent::ThreadEnvironment grows two new methods (create_sibling_thread,
  list_available_agents) with default impls that return an error, so
  existing implementors are unaffected.
• A new agent::SiblingThreadHost trait bridges the agent crate (which
  cannot depend on UI) to agent_ui. NativeAgent stores an optional host,
  NativeThreadEnvironment forwards calls to it.
• AgentPanel implements the host (AgentPanelSiblingHost) and installs it
  on the NativeAgent whenever a native-agent conversation view connects.
• AgentPanel::create_thread_with_options parameterizes the existing
  create_thread with an initial content block, agent, and model override.
  The plain create_thread becomes a thin wrapper that passes defaults, so
  no existing call sites change.

The tool goes through the same create_agent_thread → ConversationView
path as the user-facing "new thread" action, so eviction, persistence, and
sidebar rendering are identical to user-created threads.

What's not in this PR

• use_new_worktree: true currently returns an error. The existing worktree
  flow (handle_worktree_requested / open_worktree_workspace_and_start_thread)
  already creates a second Workspace inside the current MultiWorkspace
  via find_or_create_workspace; a follow-up will thread a "background"
  placement parameter through that flow so it can skip
  multi_workspace.activate and install the new thread without taking focus.
• No tool for operating on an already-created sibling thread by ID. The
  initial success payload deliberately does not include a session ID because
  the session is populated asynchronously and we have no downstream consumer
  for it today.

Testing

• cargo check --lib and cargo clippy --no-deps are clean on agent,
  agent_ui, and settings_ui.
• Manually exercised in a dev build: the agent can create a sibling thread
  with a title and prompt, the user's active thread stays focused, the new
  thread appears in the sidebar and begins generating.
• list_agents_and_models correctly enumerates the native agent with its
  default model flagged, plus any configured external agents.

Suggested .rules additions

None required — a .rules file for the agent crate was added alongside
this change documenting the non-obvious fact that a new built-in tool must
be listed in assets/settings/default.json's built-in profiles and either
added to TOOLS or EXCLUDED_TOOLS in
settings_ui/src/pages/tool_permissions_setup.rs. I hit both of those gates
during this work.

Release Notes:

• N/A

[SomeoneToIgnore]

Autoclosing all draft PRs with no activity for 2 weeks or over, this one also has conflicts:

[eholk]

Reopened as #57987 — GitHub wouldn't let me reopen this one after a force-push rewrote the branch.