Support triggering pipelines for issues

Feature request

Add an additional on-event trigger for issues. Besides pull requests, issues are generally the second place of interaction for most people on a repository. Adding the ability the automatically trigger workflows on issues open/close/etc would be greatly beneficial.

As far as I can tell all supported forges can emit webhook events for issues:
github, gitlab, bitbucket and forgejo

Use case

Github refers to this as IssueOps, but, effectively, using issues to trigger workflows. Using issues as a source of truth and an automatic kick off point for workflows (i.e., without chat triggers) is quite useful for agentic workflows.

Existing work arounds

1. Chats commands. Requires someone to trigger the workflow per issue.
2. Triggering on issues can be done partially currently using 'incoming' events and a forge-specific action, but this is counter-intuitive. It requires per repo configuration with the Repository CR and action workflow to map the issue to a webhook. This also requires a second type runners and pipelines to use the forge-specific ones, I would prefer to use tekton pipelines for all the things.

Start by tracing the existing pull-request trigger handling and the incoming-event path described in the issue, then compare the webhook event support across GitHub, GitLab, Bitbucket, and Forgejo. Define how issue open, close, and other supported actions map to workflows and what configuration is required. Done means issues can trigger pipelines without forge-specific actions or per-repository webhook mapping.