feat: add --no-wildcard flag to coder config-ssh (#26753)

Add `--no-wildcard` (`CODER_CONFIGSSH_NO_WILDCARD`) to `coder
config-ssh` that generates an individual `Host` entry per workspace
instead of a single wildcard block (`Host *.coder`).

The wildcard approach cannot be enumerated by third-party SSH clients,
the VS Code Remote-SSH sidebar, or scripts that parse `~/.ssh/config` to
discover hosts. With `--no-wildcard`, each workspace gets its own entry
so those tools work without Coder-specific extensions.

The flag is persisted in the config section header so re-running without
it prompts the user about the option change. Workspaces are fetched with
pagination before writing so the diff shows actual hostnames.

## Manual testing

**Unit tests (no server needed):**

```sh
go test ./cli/ -run TestSSHConfigOptions_writeToBuffer -v
go test ./cli/ -run TestConfigSSH_NoWildcard -v
```

**End-to-end with a dev server:**

1. Build: `go build -o ./coder .`
2. Start dev server in a separate terminal: `./scripts/develop.sh`
3. Log in: `./coder login http://localhost:3000`
4. Create two workspaces
5. Run both variants into temp files:
```sh
./coder config-ssh --no-wildcard --hostname-suffix coder --ssh-config-file /tmp/test-ssh-config --yes
./coder config-ssh --hostname-suffix coder --ssh-config-file /tmp/test-ssh-config-wildcard --yes
diff /tmp/test-ssh-config-wildcard /tmp/test-ssh-config
```

<details>
<summary>Output: <code>--no-wildcard</code></summary>

```
# ------------START-CODER-----------
# This section is managed by coder. DO NOT EDIT.
#
# You should not hand-edit this section unless you are removing it, all
# changes will be lost when running "coder config-ssh".
#
# Last config-ssh options:
# :hostname-suffix=coder
# :no-wildcard=true
#
Host coder.myworkspace
    ConnectTimeout=0
    StrictHostKeyChecking=no
    UserKnownHostsFile=/dev/null
    LogLevel ERROR
    ProxyCommand <coder> --global-config <config> ssh --stdio --ssh-host-prefix coder. %h

Host coder.myworkspace2
    ConnectTimeout=0
    StrictHostKeyChecking=no
    UserKnownHostsFile=/dev/null
    LogLevel ERROR
    ProxyCommand <coder> --global-config <config> ssh --stdio --ssh-host-prefix coder. %h

Host myworkspace.coder
    ConnectTimeout=0
    StrictHostKeyChecking=no
    UserKnownHostsFile=/dev/null
    LogLevel ERROR

Match host myworkspace.coder !exec "<coder> connect exists %h"
    ProxyCommand <coder> --global-config <config> ssh --stdio --hostname-suffix coder %h

Host myworkspace2.coder
    ConnectTimeout=0
    StrictHostKeyChecking=no
    UserKnownHostsFile=/dev/null
    LogLevel ERROR

Match host myworkspace2.coder !exec "<coder> connect exists %h"
    ProxyCommand <coder> --global-config <config> ssh --stdio --hostname-suffix coder %h
# ------------END-CODER------------
```

</details>

<details>
<summary>Output: wildcard (default)</summary>

```
# ------------START-CODER-----------
# This section is managed by coder. DO NOT EDIT.
#
# You should not hand-edit this section unless you are removing it, all
# changes will be lost when running "coder config-ssh".
#
# Last config-ssh options:
# :hostname-suffix=coder
#
Host coder.*
    ConnectTimeout=0
    StrictHostKeyChecking=no
    UserKnownHostsFile=/dev/null
    LogLevel ERROR
    ProxyCommand <coder> --global-config <config> ssh --stdio --ssh-host-prefix coder. %h

Host *.coder
    ConnectTimeout=0
    StrictHostKeyChecking=no
    UserKnownHostsFile=/dev/null
    LogLevel ERROR

Match host *.coder !exec "<coder> connect exists %h"
    ProxyCommand <coder> --global-config <config> ssh --stdio --hostname-suffix coder %h
# ------------END-CODER------------
```

</details>

<details>
<summary>diff wildcard → --no-wildcard</summary>

```diff
8a9
> # :no-wildcard=true
10c11
< Host coder.*
---
> Host coder.myworkspace
17c18
< Host *.coder
---
> Host coder.myworkspace2
21a23
>     ProxyCommand <coder> ssh --stdio --ssh-host-prefix coder. %h
23c25,31
< Match host *.coder !exec "<coder> connect exists %h"
---
> Host myworkspace.coder
>     ConnectTimeout=0
>     StrictHostKeyChecking=no
>     UserKnownHostsFile=/dev/null
>     LogLevel ERROR
>
> Match host myworkspace.coder !exec "<coder> connect exists %h"
```

</details>

Closes https://github.com/coder/coder/issues/17153 (Phase 1: CLI flag)

cli/testdata/coder_config-ssh_--help.golden

@@ -36,6 +36,11 @@ OPTIONS:
       --hostname-suffix string, $CODER_CONFIGSSH_HOSTNAME_SUFFIX
           Override the default hostname suffix.
 
+      --no-wildcard bool, $CODER_CONFIGSSH_NO_WILDCARD (default: false)
+          Generate an individual host entry for each workspace instead of a
+          wildcard host block. This allows third-party tools and SSH clients to
+          discover workspaces by reading the config file.
+
       --ssh-config-file string, $CODER_SSH_CONFIG_FILE (default: ~/.ssh/config)
           Specifies the path to an SSH config.