Add optional username filtering to PR comments

[Copilot]

The get-pr-comments solution now supports optional username filtering. When no username is provided, all comments are returned. When a username is specified, only comments from that user are included.

Changes

• API signature: Added optional username parameter to fetchPRComments(owner, repo, prNumber, username?, token?)
• Filtering: Filters both issue comments and review comments by user.login when username is provided
• CLI: Accepts optional 4th argument: get-pr-comments <owner> <repo> <prNumber> [username]
• Warning context: Displays username-specific warning when filtering without authentication token

Usage

// Get all comments (existing behavior)
const all = await fetchPRComments('owner', 'repo', 123);

// Filter by username
const filtered = await fetchPRComments('owner', 'repo', 123, 'copilot');

// With auth token
const filtered = await fetchPRComments('owner', 'repo', 123, 'copilot', token);

Note: Parameter order places username before token for more natural optional parameter progression.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Improve solution for PR comments to get all comments on PR</issue_title>
<issue_description>Improve the PR comments solution so that the username on a PR is optional. By default, it gets all comments on a PR. </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Improve solution for PR comments to get all comments on PR #50

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