Two live-test findings in one fix:
1. DockerExecutor has no repo fallback: when a project has no
repo_url, worker leaves task['repo'] unset and the container
host.py dispatch fails instantly with 'missing --repo' because
host-config-docker.json had no repo field either. Add the host
test-repo as the config default; projects with repo_url still
override it via worker's explicit --repo.
2. Worker discarded stderr from the final error_text, so every
container/host crash surfaced only as the generic 'host
execution failed' and was undebuggable from the task record.
When stdout is empty, error_text now keeps the stderr tail
(last 1000 chars, where the real error line lives).
Live test found the gap: a nonexistent model fails in
thread/start, host.py exits non-zero without JSON, so the
executor result has host_status=None and the worker never
fell back (task failed, no worker_fallback audit). When
host_status is absent, worker now extracts an explicit signal
from stdout/stderr/runner error: 404/429/5xx codes, or
'model not found' style text (counted as 404). Generic
failures with no explicit code still do not fall back, and
401 in stderr still never falls back. Tests: fallback 9 pass,
cloud 333 pass, host 27 pass.
Worker previously only recorded a model_fallback event on a
fallbackable failure and failed the task; ModelRouter.next_after
was never used to actually run the task on another model.
With --fallback-models (default empty = off, old behaviour
unchanged), a fallbackable failure (404/429/5xx/timeout; 401/400
and cancellations never fall back) now re-runs the same task,
same worktree/branch, on the first fallback model via host.py's
top-level --model flag (no host.py signature change). The second
attempt's result is final: actual_model is the fallback model,
fallback_count=1, and a double failure keeps both reasons in
error_text. LocalExecutor/DockerExecutor pass task model through
to host.py; CloudExecutor includes model only when set.
host