Dynamic per-org skill library: in-app authoring, versioning, fork (+ marketplace seam)

Skills move from a global Git-only registry to a per-company library that orgs author and
version in-app — Git stays as the shared *starter* library.

Domain & persistence:
- Skill gains OrganizationId (null = shared builtin, visible to every org), Origin
  (Builtin | Authored | Installed), AuthoredByMemberId. Identity is now
  (OrganizationId, SkillKey, Version); the unique index uses NULLS NOT DISTINCT so builtins
  stay unique by key+version while each org gets its own namespace (and can fork a builtin).
  AddSkillOwnership migration backfills existing rows as Builtin.
- Owned GoldenExample rows are cloned in Skill.Index so a fork can't re-parent the source's
  tracked entities.

Authoring (tenant, dynamic):
- POST /api/skills/authored — structured fields → same indexer pipeline (embedding +
  publish gate apply identically), tagged org + author. POST /api/skills/{key}/fork copies a
  builtin/global skill into your org as an editable Authored draft. List/Get are org-scoped
  (your org + shared builtins). New Capability.ManageSkills (Owner + TeamOwner), audited.
- GET /api/skills/marketplace: read-only seam listing public skills across orgs (install is
  the next step).

Security (from adversarial review — two confirmed criticals):
- Managing shared builtins is an operator action, not a tenant one. /index (posts arbitrary
  content as a global builtin) and /sync (re-indexes the shared library) now require a
  platform admin key (X-Skills-Admin-Key, fixed-time compare, fail-closed when unset) via
  SkillAdminOptions — previously any authenticated user of any org could inject/poison global
  skills. New test asserts an authenticated Owner without the key gets 403 on both.

UI: new /skills library page — browse shared + org skills grouped by key with their versions,
create / new-version / fork, golden-test editor + body, Draft/Published badge and the
publish-gate hint (needs roles + ≥1 golden test).

Verified: ArchitectureTests 8/8, IntegrationTests 46/46 (new SkillLibraryTests: org
isolation, version coexistence, fork, publish gate, Member 403, admin-gate 403), client build
green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/src/App.tsx

@@ -9,6 +9,7 @@ import { OrgChartPage } from '@/pages/OrgChartPage'
 import { PerformancePage } from '@/pages/PerformancePage'
 import { ReviewsPage } from '@/pages/ReviewsPage'
 import { SeatsPage } from '@/pages/SeatsPage'
+import { SkillsPage } from '@/pages/SkillsPage'
 import { StructurePage } from '@/pages/StructurePage'
 import { useAuth } from '@/store/auth'
 
@@ -27,6 +28,7 @@ export default function App() {
         <Route path="/members" element={token ? <MembersPage /> : <Navigate to="/login" replace />} />
         <Route path="/org" element={token ? <OrgChartPage /> : <Navigate to="/login" replace />} />
         <Route path="/structure" element={token ? <StructurePage /> : <Navigate to="/login" replace />} />
+        <Route path="/skills" element={token ? <SkillsPage /> : <Navigate to="/login" replace />} />
         <Route path="/performance" element={token ? <PerformancePage /> : <Navigate to="/login" replace />} />
         <Route path="*" element={<Navigate to="/" replace />} />
       </Routes>