chore: remove agents experiment flag and mark feature as beta (#24432)

Remove the `ExperimentAgents` feature flag so the Agents feature is
always available without requiring `--experiments=agents`. The feature
is now in beta.

Existing deployments that still pass `--experiments=agents` will get a
harmless "ignoring unknown experiment" warning on startup.

### Changes

**Backend:**
- Remove `RequireExperimentWithDevBypass` middleware from chat and MCP
server routes
- Always include `AgentsAccessRole` in assignable site roles (later
refactored to org-scoped on main; rebase keeps that)
- Always set `AgentsTabVisible = true`, then drop the entire dead
`AgentsTabVisible` metadata pipeline (Go htmlState field,
populateHTMLState goroutine, HTML meta tag, useEmbeddedMetadata
registration, mock); no production consumer reads it. `AgentsNavItem`
already gates on `permissions.createChat`.
- Make `blob:` CSP `img-src` addition unconditional
- Remove `ExperimentAgents` constant, `DisplayName` case, and
`ExperimentsKnown` entry

**CLI:**
- Graduate the agents TUI from `coder exp agents` to `coder agents`
(moved from `AGPLExperimental()` to `CoreSubcommands()`)
- Drop the `agent` alias so it does not collide with the hidden
workspace-agent command
- Rename implementation files `cli/exp_agents_*.go` -> `cli/agents_*.go`
and internal identifiers (`expChatsTUIModel` -> `chatsTUIModel`,
`newExpChatsTUIModel` -> `newChatsTUIModel`, `setupExpAgentsBackend` ->
`setupAgentsBackend`, `startExpAgentsSession` -> `startAgentsSession`,
`expAgentsPtr` -> `agentsPtr`, `expAgentsSession` -> `agentsSession`,
`TestExpAgents*` -> `TestAgents*`). `expClient` (the
`*codersdk.ExperimentalClient` local) is kept; `coderd/exp_chats*.go`
and other still-experimental `cli/exp_*.go` commands are intentionally
untouched.

**Frontend:**
- Remove experiment check from `AgentsNavItem` - render when
`canCreateChat` is true
- Remove `agentsEnabled` experiment check from `WorkspacesPage`, then
gate `chatsByWorkspace` on `permissions.createChat` so users without
chat access don't trigger the per-page DB query (Copilot review
feedback)
- Add `FeatureStageBadge` (beta) next to the Coder logo in the Agents
sidebar (desktop + mobile)

**Docs:**
- Remove experiment flag setup instructions from `early-access.md` and
`getting-started.md` (and rename `early-access.md`'s "Enable Coder
Agents" heading to "Set up Coder Agents", since there is no enablement
step left)
- Update `chats-api.md` and `getting-started.md`'s Chats API note to say
"beta" instead of "experimental"
- `docs/manifest.json`: drop "experimental" from the Chats API sidebar
description
- `make gen` regenerated `docs/reference/cli/agents.md` and the CLI
index
- `scripts/check_emdash.sh`: exclude `cli/testdata/*.golden` and
`enterprise/cli/testdata/*.golden` from the new repo-wide emdash lint,
since serpent emits emdash borders in every generated `--help` golden
file

**Tests:**
- Remove `ExperimentAgents` setup from all test files (14 occurrences
across 7 files)
- Update stale "with the agents experiment" comments in
`coderd/x/chatd/integration_test.go` and `coderd/mcp_test.go`


<img width="1185" height="900" alt="image"
src="https://github.com/user-attachments/assets/b420bc8f-41d6-42c6-abd8-ad572533d651"
/>


> 🤖 Generated by Coder Agents

coderd/coderd.go

@@ -768,7 +768,7 @@ func New(options *Options) *API {
 	}
 	api.agentProvider = stn
 
-	{ // Experimental: agents — chat daemon and git sync worker initialization.
+	{ // Chat daemon and git sync worker initialization.
 		maxChatsPerAcquire := options.DeploymentValues.AI.Chat.AcquireBatchSize.Value()
 		if maxChatsPerAcquire > math.MaxInt32 {
 			maxChatsPerAcquire = math.MaxInt32
@@ -1153,11 +1153,9 @@ func New(options *Options) *API {
 				})
 			})
 		})
-		// Experimental(agents): chat API routes gated by ExperimentAgents.
 		r.Route("/chats", func(r chi.Router) {
 			r.Use(
 				apiKeyMiddleware,
-				httpmw.RequireExperimentWithDevBypass(api.Experiments, codersdk.ExperimentAgents),
 			)
 			r.Get("/by-workspace", api.chatsByWorkspace)
 			r.Get("/", api.listChats)
@@ -1280,7 +1278,6 @@ func New(options *Options) *API {
 			)
 			// MCP server configuration endpoints.
 			r.Route("/servers", func(r chi.Router) {
-				r.Use(httpmw.RequireExperimentWithDevBypass(api.Experiments, codersdk.ExperimentAgents))
 				r.Get("/", api.listMCPServerConfigs)
 				r.Post("/", api.createMCPServerConfig)
 				r.Route("/{mcpServer}", func(r chi.Router) {
@@ -2006,14 +2003,10 @@ func New(options *Options) *API {
 			"parsing additional CSP headers", slog.Error(cspParseErrors))
 	}
 
-	// Add blob: to img-src for chat file attachment previews when
-	// the agents experiment is enabled.
-	if api.Experiments.Enabled(codersdk.ExperimentAgents) {
-		additionalCSPHeaders[httpmw.CSPDirectiveImgSrc] = append(
-			additionalCSPHeaders[httpmw.CSPDirectiveImgSrc], "blob:",
-		)
-	}
-
+	// Add blob: to img-src for chat file attachment previews.
+	additionalCSPHeaders[httpmw.CSPDirectiveImgSrc] = append(
+		additionalCSPHeaders[httpmw.CSPDirectiveImgSrc], "blob:",
+	)
 	// Add CSP headers to all static assets and pages. CSP headers only affect
 	// browsers, so these don't make sense on api routes.
 	cspProxyHosts := func() []*proxyhealth.ProxyHost {
@@ -2161,9 +2154,9 @@ type API struct {
 	// dbRolluper rolls up template usage stats from raw agent and app
 	// stats. This is used to provide insights in the WebUI.
 	dbRolluper *dbrollup.Rolluper
-	// Experimental(agents): chatDaemon handles background processing of pending chats.
+	// chatDaemon handles background processing of pending chats.
 	chatDaemon *chatd.Server
-	// Experimental(agents): gitSyncWorker refreshes stale chat diff statuses in the background.
+	// gitSyncWorker refreshes stale chat diff statuses in the background.
 	gitSyncWorker *gitsync.Worker
 	// AISeatTracker records AI seat usage.
 	AISeatTracker aiseats.SeatTracker