Document playwrightCliVersion strict SemVer validation

Document the playwrightCliVersion configuration setting and its strict
SemVer 2.0 requirement, introduced in microsoft/aspire#18205.

The Playwright CLI installer now validates that playwrightCliVersion
contains a MAJOR.MINOR.PATCH version string before forwarding it to npm.
Dist-tags (latest), ranges (>=1.0.0), and v-prefixed forms (v1.52.0)
are all rejected with a descriptive error message.

Add a 'Pin the Playwright CLI version' section to aspire-skills.mdx
covering the configuration key, valid format, and the new error message
users may see if their config value does not pass validation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: aspire-repo-bot[bot]

src/frontend/src/content/docs/get-started/aspire-skills.mdx

@@ -256,6 +256,30 @@ Use `dotnet-inspect` only when an agent needs to inspect .NET code or packages o
 
 The `playwright-cli` skill works best alongside the `aspire` skill. The agent will first use Aspire to discover the running app and the correct frontend endpoint, especially when multiple web resources exist. After it has the target URL, it can use the Playwright CLI to automate browser testing.
 
+## Pin the Playwright CLI version
+
+By default, `aspire agent init` resolves the Playwright CLI version from the npm registry according to the range bundled with your installed Aspire CLI. You can override this with an exact version by setting `playwrightCliVersion` in your `aspire.config.json`:
+
+```json title="aspire.config.json"
+{
+  "playwrightCliVersion": "1.52.0"
+}
+```
+
+Or via the CLI:
+
+```bash title="Aspire CLI"
+aspire config set playwrightCliVersion 1.52.0
+```
+
+The value must be a valid [SemVer 2.0](https://semver.org/spec/v2.0.0.html) version in strict `MAJOR.MINOR.PATCH` form. Values that are not valid strict SemVer — such as dist-tags (`latest`), npm version ranges (`>=1.0.0`), or prefixed versions (`v1.52.0`) — are rejected with a clear error message:
+
+```text title="Error"
+The value of configuration setting 'playwrightCliVersion' ('latest') is not a valid SemVer 2.0 version.
+```
+
+Correct the value to an exact version such as `1.52.0` and re-run `aspire agent init`.
+
 ## Skill locations
 
 Aspire installs each selected skill into the selected skill locations. For example, a standard location can contain every Aspire workflow skill and selected companion skills: