fix: allow pivots from failed task evidence

What

Allow a follow-up task to cite an observation produced by a failed task and use that task as an evidence dependency.

Why

Blocked probes can still produce useful negative results. Requiring every dependency to be DONE currently prevents the Supervisor from pivoting on that evidence.

Verification

• uv run python -m pytest -q — 119 passed, 1 skipped
• Ruff and mypy passed
• git diff --check passed

Fixes #568