feat(dashboard): design self-host review config generator

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Context

Self-hosters should eventually be able to configure review behavior from the website/admin dashboard instead of hand-editing YAML. The dashboard must generate or write the same config values consumed by .gittensory.yml / GITTENSORY_REPO_CONFIG_DIR, not create a second source of truth.

Parent: #1680

Requirements

• Design the dashboard workflow for selecting repo, gate policy, review profile, inline comments, RAG, REES analyzers, AI provider mode, and autonomy level.
• Make the generated output explicit and reviewable before it is saved or copied.
• Preserve private-config boundaries: private maintainer policy must not leak into public repo-root files.
• Define validation errors using the existing parser/schema so dashboard and YAML behavior match.
• Include import/export paths for existing private repo config.

Deliverables

• Product/design spec for the dashboard flow.
• Data model/API plan reusing existing config parser and private config directory behavior.
• Security notes for auth, repo access, secret handling, and public/private config separation.
• Implementation breakdown for a later UI/API PR.

Acceptance criteria

• The dashboard plan has one config source of truth and no parallel schema.
• Generated config can be diffed/copied and validates against the same parser as file-based config.
• The private config workflow remains safe for self-hosted maintainers.
• No labels beyond maintainer-only.

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2026-07-01 audit: consolidated duplicate #1673 here; canonical dashboard is #1967.