Do not retain a previous reconstruction report after a failed run #111

At PR #20 head d072729, recon/recon/pipeline.py:127-147 writes new model, geometry, scene and coverage files before calling the server. If server.place() raises, execution never reaches the new report write. A reused output directory retains the preceding run's report.md beside the new artifacts and refusal body.

Clear the previous report before a new run can fail, or publish the full output set atomically. Test a reused output directory with an existing report and a mocked server failure, without reconstruction or model jobs. The final directory must not describe a previous reconstruction as the current result.

Confirmed P2 follow-up under the manager's convergence rule. The worker remains an unused draft handoff.

Review: #20 (comment)