feat(hub): org-scoped agent role/skill folder tree (ADR-0028)

Add a shared transparent OrganizationAgentConfigFolder tree for grouping
agent roles and skills in the admin UI without affecting identity, bindings,
run loading, or slash commands.

docs/adr/0028-agent-config-folder-tree.md

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+# ADR 0028: Agent Configuration Folder Tree
+
+## Status
+
+Accepted.
+
+## Context
+
+ADR-0017/0018 made Agent roles and skills Organization-scoped dynamic runtime
+configuration, managed without process restarts. The org admin surfaces for
+them (`/admin/roles` and `/admin/skills`) render every
+role/skill as one large editor card in a single flat list. As an Organization
+accumulates roles and skills, the management pages degrade into an endless
+scroll with no grouping affordance.
+
+The project explorer already solves the analogous problem for projects with
+transparent folders (ADR-0021): org-scoped navigation nodes that are not
+permission resources. Roles and skills need the same affordance, but their
+semantics differ from projects in one crucial way: skill names and role IDs
+are referenced by role→skill bindings, run-time skill snapshot loading, and
+Feishu slash commands. Any grouping mechanism must not leak into those
+resolution paths.
+
+## Decision
+
+Introduce an Organization-scoped folder tree shared by Agent roles and Agent
+skills:
+
+- One folder tree per Organization is shared by both roles and skills (e.g. a
+  "高三化学组" folder groups that team's roles and its skills together). It is
+  a distinct entity from the project explorer `Folder` of ADR-0021 — the two
+  trees are managed independently and never reference each other.
+- Folders are **transparent organization nodes**, following the ADR-0021
+  project-folder precedent: they exist for management-surface navigation and
+  grouping only, are not permission resources, and hold no grants.
+- Folder membership is **not part of role/skill identity or resolution**:
+  - skill `name` and role `roleId` remain unique per Organization regardless
+    of folder membership;
+  - role→skill bindings, run admission's frozen role snapshot, run-scoped
+    skill loading, and Feishu slash commands never reference folders.
+- Each role/skill sits in at most one folder; membership is optional
+  (unfiled items remain first-class). Folders nest arbitrarily.
+- Folder assignment is a label-class change in the ADR-0017 sense: it never
+  archives Agent sessions, because the execution surface (model, prompt,
+  tools, skill content) is untouched.
+- A folder can be deleted only when empty — no child folders, no roles, no
+  skills. Relocating items out of a folder is an explicit user action, so no
+  orphan-placement rule is needed yet.
+- Admin web renders both pages as a left folder tree plus the item list of
+  the selected folder ("all" and "unfiled" included). The host-console CLI is
+  unchanged: folder management lives in the web surface, and CLI
+  `upsert-role`/`install-skill` never touch folder assignment.
+
+## Consequences
+
+- New DB entity `OrganizationAgentConfigFolder` (org-scoped, self-nesting via
+  `parentId`, delete restricted while referenced) plus nullable `folderId` on
+  `OrganizationAgentRole` and `OrganizationAgentSkill` (SetNull on folder
+  delete, though the service refuses to delete non-empty folders).
+- The spec pins the transparency and single-membership semantics in
+  `Spec.System.AgentRole` (`AgentConfigFolder`), so future implementors do
+  not re-derive them differently (e.g. path-style names or per-folder
+  uniqueness).
+- Org admin APIs gain folder CRUD plus role/skill folder-assignment endpoints
+  that skip session archival by construction.
+- Moving a role/skill between folders changes nothing about authorization,
+  run resolution, or audit-visible configuration lineage beyond the folder
+  assignment event itself.
+
+## Open Questions / Deferred
+
+- Drag-and-drop assignment and bulk moves are deferred; assignment is a
+  per-item select for now.
+- Folder-level usage aggregation for roles/skills is deferred (project
+  folders already aggregate usage under ADR-0021; agent configuration has no
+  usage dimension yet).
+- CLI flags for folder assignment are deferred until a console workflow asks
+  for them.
+- Folder-scoped default-role policies (e.g. per-folder defaults) are rejected
+  for now: the Organization keeps exactly one active default role
+  (ADR-0017/0018 invariant) regardless of folder structure.