* fix: make repository optional when starting a dashboard run
The agent infers and clones the target repo from the task itself, so a
default repo is never actually required to run — but the dashboard 400'd
("no default repository configured") when a user had none set.
Treat repo as optional: _resolve_repo_config returns {} instead of
raising, repo metadata/config are only written when a repo is present,
and the "missing repository metadata" gate on follow-up messages is
dropped. UI hides the repo chip when absent.
* feat: add repo picker to the run prompt bar
Adds an optional, searchable repository selector next to the model picker
on the Agents home prompt bar (Cursor-style). It pre-fills the user's saved
default repo and can be cleared to "No repository" for a repo-less run.
Because the picker now resolves the default on the client, the create
endpoint honors the request value verbatim: _resolve_repo_config just parses
what's sent ({} when empty) instead of falling back to the saved default,
so an explicit "No repository" is respected.
* refactor: match Cursor layout for repo placement
Move the repo selector out of the prompt-box footer to a pill row above
the input (folder + caret, dropdown opens downward); the model picker
stays inside the box. In the thread view, show the thread title and repo
in a header at the top of the chat, with the follow-up input pinned to
the bottom as before.
