Link Jira tickets to threads (similarly to how PRs are linked).

[Jardo-51]

In a corporate environment, everything revolves around Jira tickets. Most of my threads at work are related to a Jira ticket, often having multiple threads for a single Jira ticket.

I want to be able to:

• quickly identify which Jira ticket a thread is related to
• find all threads related to a Jira ticket (even old settled ones)
• have a link to quickly open the Jira ticket from the thread

Proposed solution

Implement Jira ticket linking similar to the current PR linking:

• When the agent recognizes a thread is related to a Jira ticket, it should automatically link it to the thread
• Display the Jira ticket number as a link under the composer (next to the MR link, there is plenty of space)
• Display the Jira ticket number as a link in the sidebar: this is a little trickier, there is not so much space. There could be a setting that enables the user to choose if they want to see the PR link, or the ticket link (or both, if they have enough space).
• Extend the search functionality to find threads by the ticket number (although this is not currently implemented for PRs)

[juliusmarminge]

Duplicate of #11785 — that Ideas thread already covers linking threads to Jira (and other trackers) like PRs: show the ticket on the thread, search by identifier, and open it from the linked-work surfaces. Closing this one so the ask stays in one place; feel free to add any Jira-specific UI notes (composer vs sidebar) on #11785.