feat(cli): add coder exp mcp command (#17066)

Adds a `coder exp mcp` command which will start a local MCP server
listening on stdio with the following capabilities:
* Show logged in user (`coder whoami`)
* List workspaces (`coder list`)
* List templates (`coder templates list`)
* Start a workspace (`coder start`)
* Stop a workspace (`coder stop`)
* Fetch a single workspace (no direct CLI analogue)
* Execute a command inside a workspace (`coder exp rpty`)
* Report the status of a task (currently a no-op, pending task support)

This can be tested as follows:

```
# Start a local Coder server.
./scripts/develop.sh
# Start a workspace. Currently, creating workspaces is not supported.
./scripts/coder-dev.sh create -t docker --yes
# Add the MCP to your Claude config.
claude mcp add coder ./scripts/coder-dev.sh exp mcp
# Tell Claude to do something Coder-related. You may need to nudge it to use the tools.
claude 'start a docker workspace and tell me what version of python is installed'
```

cli/exp.go

@@ -13,6 +13,7 @@ func (r *RootCmd) expCmd() *serpent.Command {
 		Children: []*serpent.Command{
 			r.scaletestCmd(),
 			r.errorExample(),
+			r.mcpCommand(),
 			r.promptExample(),
 			r.rptyCommand(),
 		},