[Story]: Admin UI for managing content-exclusion lists

## Summary
Build an admin UI/API surface for managing content-exclusion lists, so platform and scoped admins can add/remove excluded repos and paths without direct database access.

## Intent / Source of truth
The exclusion feature needs to be operable by customer admins, not just engineers with API access, to be a real product feature rather than an internal escape hatch. Part of [Epic] Content and path exclusion controls.

## Scope
- [ ] Admin UI list/add/remove exclusion rules
- [ ] Scoped-admin visibility (an admin sees/manages exclusions within their subtree, once the org-hierarchy epic ships)
- [ ] Bulk import of exclusion patterns

## Out of scope
- End-user (non-admin) visibility into exclusion state

## Verification
UI walkthrough covering add/remove/bulk-import; scoped-admin restriction test once org hierarchy is available (otherwise falls back to platform-admin-only).

## Risk assessment
Low risk; primarily a UI/API convenience layer on top of a6s1's data model.

## AI Usage Declaration
Drafted with AI assistance during the 2026-08-13 cross-repo backlog consolidation and enterprise-readiness research. A human owns intent, verification and consequences.

Start from a6s1's data model and the requested admin UI/API surface. Done means a UI walkthrough covers adding, removing, and bulk-importing exclusion rules, with scoped-admin restriction coverage once the org hierarchy is available, or platform-admin-only behavior otherwise.

索引モデルが issue の本文から書いたものです。