feat(maintainers): add repo settings preview and install diagnostics

[JSONbored]

Problem

Maintainers need to know exactly what the GitHub App will do before enabling comments, labels, or optional checks. Today install health exists, but the maintainer-facing preview path can be clearer.

Desired behavior

Add a settings/diagnostics API that shows public surface mode, label settings, missing permissions, check mode, maintainer-author inclusion, linked-issue requirements, and a dry-run preview for a sample PR.

Implementation notes

• Reuse existing installation health and repo settings concepts.
• Add a preview endpoint that renders the sanitized comment/label decision without posting anything.
• Explain missing Issues: write and optional Checks: write remediation exactly.
• Keep check runs off by default unless explicitly configured.

Public/private boundaries

The preview must use the same sanitizer as public GitHub output. Private scoreability, raw trust, wallet/hotkey, and reviewability internals must not appear.

Acceptance criteria

• Maintainer can see whether a sample PR would be skipped, commented, labeled, or minimally checked.
• Missing permissions are explained with exact GitHub App setting changes.
• Confirmed miner, non-miner, bot, maintainer author, disabled surface, and missing permission cases are handled.
• Preview does not mutate GitHub state.

Tests

• Healthy install.
• Missing Issues: write.
• Optional checks disabled.
• Confirmed miner preview.
• Non-miner quiet skip preview.
• Maintainer-author default skip.
• Sanitizer regression.