Add option to suppress @mention in completion comment

## Problem

When a tag-mode job completes, the action posts a completion comment that always `@mentions` the triggering user:

```
**Claude finished @username's task in 1m 2s** —— View job
```

This creates an unnecessary notification for the user who triggered the job — they already get notified by GitHub when the workflow completes. The `@mention` in the comment body adds a duplicate ping.

## Relevant code

[`src/github/operations/comment-logic.ts:127`](https://github.com/anthropics/claude-code-action/blob/main/src/github/operations/comment-logic.ts#L127):

```ts
header = `**Claude finished @${username}'s task`;
```

## Proposed solution

Add an input parameter (e.g. `suppress_completion_mention: true`) that replaces the `@username` mention with a plain-text username in the completion header:

```
**Claude finished username's task in 1m 2s** —— View job
```

This preserves the useful completion info while avoiding the extra notification.

## Alternatives

- A broader `completion_comment: false` flag to suppress the entire completion comment
- Making the header template configurable via an input parameter

Start in src/github/operations/comment-logic.ts around line 127, then trace how action inputs are parsed and passed to completion-comment handling. Done means the new option produces a plain-text username in the completion header while preserving the rest of the comment; the issue does not name a test file to run.

Written by the indexing model from the issue text.