feat: make Coder Agents right sidebar app and port tabs generally available (#26906)

The workspace-app and port preview tabs in the Coder Agents right panel
were gated behind the `agent-app-tabs` deployment experiment. This
removes the experiment entirely and renders the app and port tabs
unconditionally, so the add-panel dropdown, workspace-app tabs, and port
preview tabs are always available alongside terminals.

## Changes

- Remove the `ExperimentAgentAppTabs` constant, its `DisplayName()`
case, and its `ExperimentsKnown` registration in
`codersdk/deployment.go`, then regenerate
`site/src/api/typesGenerated.ts`, `coderd/apidoc/docs.go`,
`coderd/apidoc/swagger.json`, and `docs/reference/api/schemas.md`.
- Drop the frontend experiment gate in `AgentChatPageView.tsx`
(including the now-unused `useDashboard`/`experiments` usage) so
persisted app and port tabs are no longer filtered out.
- Remove the `appExperimentEnabled` prop from `RightPanelAddTabControl`
and render the add-panel dropdown unconditionally; update the stories
accordingly.

This reverses the gating introduced in #26395.

note: the diff is tiny if you hide whitespace changes

site/src/api/typesGenerated.ts

@@ -4586,7 +4586,6 @@ export const EntitlementsWarningHeader = "X-Coder-Entitlements-Warning";
 // From codersdk/deployment.go
 export type Experiment =
 	| "ai-gateway-cost-control"
-	| "agent-app-tabs"
 	| "auto-fill-parameters"
 	| "chat-advisor"
 	| "chat-virtual-desktop"
@@ -4601,7 +4600,6 @@ export type Experiment =
 
 export const Experiments: Experiment[] = [
 	"ai-gateway-cost-control",
-	"agent-app-tabs",
 	"auto-fill-parameters",
 	"chat-advisor",
 	"chat-virtual-desktop",