refactor: remove AI add-on badge, seat column, and add-on wording leftovers (#28004)

Removes all "AI add-on" labeling from the UI, plus two leftover "AI
Governance add-on" wording items from the licensing repackaging (AI
Governance is now included with Premium).

- Remove the **AI add-on** badge from the group settings AI budget
section and the AI Governance add-on card on the Licenses page.
- Remove the **AI add-on** column (check icon showing AI seat
consumption) from the Users and Organization Members tables, along with
its help popover, the `AISeatCell` component, and the
`shouldShowAISeatColumn` entitlement helper.
- Update related Storybook stories.
- Drop "add-on" from the AI cost control route comments in
`enterprise/coderd/coderd.go` and the `coder exp ai-model-prices`
prerequisite in `docs/ai-coder/ai-gateway/cost-controls.md` (leftovers
not covered by #28075/#28077/#28268).

The `has_ai_seat` API field is left intact.

<details>
<summary>Removal scope</summary>

Badge usages removed:

- `site/src/pages/GroupsPage/GroupSettingsPageView.tsx`
-
`site/src/pages/DeploymentSettingsPage/LicensesSettingsPage/AIGovernanceAddOnCard.tsx`

AI seat column plumbing removed:

- `site/src/pages/UsersPage/UsersTable.tsx` and `UsersPage.tsx`
- `site/src/pages/OrganizationSettingsPage/OrganizationMembersTable.tsx`
and `OrganizationMembersPage.tsx`
- `site/src/modules/users/AISeatCell.tsx` (deleted)
- `AiAddonHelpPopover` in `site/src/modules/users/UserHelpPopovers.tsx`
- `shouldShowAISeatColumn` in
`site/src/modules/dashboard/entitlements.ts`

Add-on wording leftovers:

- `enterprise/coderd/coderd.go`: 7 route comments (comments only, no
behavior change)
- `docs/ai-coder/ai-gateway/cost-controls.md`: model prices CLI
prerequisite

</details>

---

🤖 This pull request was generated by Coder Agents on behalf of
@tracyjohnsonux.

docs/ai-coder/ai-gateway/cost-controls.md

@@ -217,9 +217,9 @@ price for it yourself.
 ### Set model prices
 
 Use the experimental `coder exp ai-model-prices` command to set prices for
-models the price book does not cover. It requires the AI Governance add-on and
-the `ai_model_price:update` permission. Run
-`coder exp ai-model-prices --help` for the full reference.
+models the price book does not cover. It requires AI Governance, which is
+included with a Premium license, and the `ai_model_price:update` permission.
+Run `coder exp ai-model-prices --help` for the full reference.
 
 List the prices this deployment holds, optionally narrowed to one provider or
 model: