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

cli/testdata/coder_--help.golden

@@ -14,6 +14,7 @@ USAGE:
        $ coder templates init
 
 SUBCOMMANDS:
+    agents            Interactive terminal UI for AI agents.
     autoupdate        Toggle auto-update policy for a workspace
     completion        Install or update shell completion scripts for the
                       detected or chosen shell.

cli/testdata/coder_agents_--help.golden

@@ -0,0 +1,16 @@
+coder v0.0.0-devel
+
+USAGE:
+  coder agents [flags] [chat-id]
+
+  Interactive terminal UI for AI agents.
+
+OPTIONS:
+      --model string
+          Choose a model by ID, provider/model, or display name.
+
+      --workspace string
+          Associate the chat with a workspace by name, owner/name, or UUID.
+
+———
+Run `coder --help` for a list of global options.