Add @bot coverage comment trigger to coverage workflow

[Copilot]

Enable coverage workflow invocation via @phlexbot coverage comment on PRs, bypassing relevance detection.

Changes

• Added issue_comment trigger: Workflow responds to PR comments matching @{repo}bot coverage pattern
• Authorization check: Restricted to OWNER/COLLABORATOR/MEMBER via github.event.comment.author_association check
• PR context retrieval: Integrated get-pr-info action to extract ref/repo/base_sha when comment-triggered
• Bypass relevance detection: Skip detect-changes job for issue_comment events (same behavior as workflow_dispatch)

Pattern

Follows the established pattern from cmake-build.yaml, python-fix.yaml, and other bot-triggered workflows.

Security

CodeQL TOCTOU alerts are expected—workflow executes code from PR head after authorization check. This is the intentional security model for bot-triggered workflows (same as existing build/fix workflows).

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[Copilot]

Pull request overview

This PR adds the ability to trigger the coverage workflow via @phlexbot coverage comment on pull requests, following the established bot command pattern used in other workflows like cmake-build and python-fix.

Changes:

• Added issue_comment event trigger with authorization check restricting to OWNER/COLLABORATOR/MEMBER
• Integrated get-pr-info action to extract PR context (ref, repo, base_sha) when comment-triggered
• Modified detect-changes job to skip for issue_comment events (same as workflow_dispatch)