chore: remove IS_AI_ENABLED feature flag (#19916)

## Summary
- AI is now GA, so the public/lab `IS_AI_ENABLED` flag is removed from
`FeatureFlagKey`, the public flag catalog, and the dev seeder.
- Drops every backend `@RequireFeatureFlag(IS_AI_ENABLED)` guard (agent,
agent chat, chat subscription, role-to-agent assignment, workflow AI
step creation) and the now-unused `FeatureFlagModule`/`FeatureFlagGuard`
wiring in the AI and workflow modules.
- Removes frontend gating from settings nav, role
permissions/assignment/applicability, command menu hotkeys, side panel,
mobile/drawer nav, and the agent chat provider so AI UI is always on.
Tests and generated GraphQL/SDK schemas updated accordingly.

## Test plan
- [x] `npx nx typecheck twenty-shared`
- [x] `npx nx typecheck twenty-server`
- [x] `npx nx typecheck twenty-front`
- [x] `npx nx lint:diff-with-main twenty-server`
- [x] `npx nx lint:diff-with-main twenty-front`
- [x] `npx jest --config=packages/twenty-server/jest.config.mjs
feature-flag`
- [x] `npx jest --config=packages/twenty-server/jest.config.mjs
workspace-entity-manager`
- [ ] Manual smoke test: AI features still accessible without any flag
row in `featureFlag`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/twenty-server/src/engine/twenty-orm/entity-manager/workspace-entity-manager.spec.ts

@@ -232,7 +232,6 @@ describe('WorkspaceEntityManager', () => {
       featureFlagsMap: {
         IS_UNIQUE_INDEXES_ENABLED: false,
         IS_JSON_FILTER_ENABLED: false,
-        IS_AI_ENABLED: false,
         IS_MARKETPLACE_SETTING_TAB_VISIBLE: false,
         IS_RECORD_PAGE_LAYOUT_EDITING_ENABLED: false,
         IS_PUBLIC_DOMAIN_ENABLED: false,
@@ -262,7 +261,6 @@ describe('WorkspaceEntityManager', () => {
       featureFlagMap: {
         IS_UNIQUE_INDEXES_ENABLED: false,
         IS_JSON_FILTER_ENABLED: false,
-        IS_AI_ENABLED: false,
         IS_PUBLIC_DOMAIN_ENABLED: false,
         IS_EMAILING_DOMAIN_ENABLED: false,
       },