feat(jira): Jira webhook trigger

[peteroden]

What

Add a POST /jira-webhook endpoint as an alternative to polling for instant Jira issue triggering. Requires Jira admin access to configure. Should use the same CodingTaskHandler interface as the poller.

Acceptance Criteria

• POST /jira-webhook endpoint with HMAC-SHA256 validation
• Parses jira:issue_updated events with status change to trigger status
• Calls the same CodingTaskHandler as the poller
• Works alongside polling (either or both can be active)