feat(app): add weekly value report export for operators and maintainers

[JSONbored]

Parent phase: #237
Parent roadmap: #127

Problem

The operator dashboard shows a weekly value-report concept, but v1 needs exportable reports that maintainers/operators can actually use to evaluate noise reduction and adoption.

Acceptance criteria

• Report can be exported in a copy-ready Markdown or JSON format.
• Report separates miner utility, maintainer trust, repo-owner readiness, adoption metrics, and known blockers.
• Exported public/shareable text is sanitized and avoids reward/score-prediction claims.

Validation expected

• API/unit tests cover report generation and sanitizer boundaries.
• Screenshots show export controls if UI is touched.

UI evidence gate

Any visible web, browser-extension, or GitHub-overlay change must include maintainer-reviewable screenshots or a short recording covering the changed states. A checked template box without actual visual evidence is not enough.

Public-output safety criteria

• Public text is sanitized before reaching GitHub comments, issue bodies, PR bodies, extension-visible public panels, or copied public snippets.
• Tests cover forbidden wallet/hotkey, reward-estimate, trust-score, public-score-prediction, private-reviewability, private-scoreability, and farming-language leakage.

Cross-cutting acceptance criteria

• Preserve the repo quality gate: npm run test:ci, 97%+ global coverage, and the local branch coverage target for touched code.
• Keep public/private boundaries explicit. Public GitHub output must not expose wallets, hotkeys, reward estimates, raw trust scores, public score predictions, private reviewability, private scoreability context, or farming language.
• Add/update focused tests for the changed behavior instead of relying on green checks alone.