[Feature]: Filter sidebar threads by who started them #17401

Area

apps/web, apps/mobile

Problem or use case

Threads started by agents now outnumber the ones I start myself: outside agents launching
through /mcp, delegate_task children, scheduled tasks. The thread view already marks a
thread as started by an agent, but the sidebar can only be narrowed by project (and
environment on mobile, #8948 for desktop). There's no way to see "only threads I started"
or "only threads an agent started" without opening each one.

Proposed solution

Add a "Started by" filter next to the project filter in the sidebar:

• Me (createdBy: user)
• Agents (createdBy: agent), optionally split by source: outside agent (mcp), scheduled task, delegated task
• All (default)

Both fields already live on every thread (createdBy, creationSource), so this is a client-side filter
with no server changes. The choice should persist across navigation (cf. #9379).

Optional follow-up: record which outside agent (its OAuth client name) launched a thread and
show it on the thread, so several outside agents can be told apart. Today every outside launch
is recorded as creationSource: "mcp".

Happy to open PR.

Alternatives considered

Collapsing agent-started threads into a group, like subagents under their parent. A filter is
simpler and also works for search results.

Additional context

Related: #1043 (status filters, closed), #3523, #8948.