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>

tests/TeamUp.IntegrationTests/AnyRoleSeatTests.cs

@@ -122,6 +122,7 @@ public sealed class AnyRoleSeatTests(PostgresFixture postgres) : IClassFixture<P
     {
         var client = factory.CreateClient();
         client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", token);
+        client.DefaultRequestHeaders.Add("X-Skills-Admin-Key", TeamUpWebFactory.PlatformAdminKey);
         return client;
     }