[Harness Watch] P1 — diagnose confirmed trajectory differences

Parent epic: #4627

Goal

After the deterministic comparison framework is stable, explain selected confirmed differences and propose one human-reviewable experiment. This does not block P0.

Reuse or refactor the existing propose-harness action instead of building a second diagnosis platform.

P1

• Consume the paired report and only the leading outcome-discordant or resource-divergent trajectories.
• Run read-only with no repository, branch, push, or issue-write capability.
• Produce a structured hypothesis, alternatives, evidence and counterevidence with instance/event links, predicted observable change, and one single-variable experiment.
• Ground quantitative statements in the deterministic report or tool output.
• Stop at the proposal; a human decides whether to implement or run it.

The agent interprets evidence. It does not calculate statistics, decide significance, or modify repositories.

Start with parent epic #4627 and the existing propose-harness action, then review how the paired report exposes outcome-discordant or resource-divergent trajectories. Done means a read-only proposal containing hypotheses, alternatives, evidence and counterevidence with instance/event links, predicted observable changes, and one single-variable experiment, without calculating statistics or modifying repositories.