[Feature]: Link (or create) a Linear issue from the workspace details/update dialog

### Problem or use case

The workspace details dialog ("Edit Worktree Details" / workspace notes) lets me attach a **GitHub Issue** and a **GitHub PR** to a workspace at any time. But there is **no Linear equivalent there** — I can only link a Linear issue when I *create* the workspace (via the new-workspace composer's source picker).

So if I create a workspace first and decide to associate it with a Linear issue later, I'm stuck: there's no post-creation UI to register an existing Linear issue, and no way to spin up a new Linear issue from the workspace. Linear is a first-class task source for me, and it's second-class compared to GitHub in the workspace surface.

### Proposed solution

- [ ] Add a **Linear issue** field to the workspace details/update dialog, alongside the existing GitHub Issue / PR fields, so I can **link an existing Linear issue to an already-created workspace** at any time.
- [ ] Optionally, let me **create a new Linear issue** from that surface and link it to the current workspace in one step.
- [ ] Reflect the link the same way creation-time linking does (the `Linked Linear {identifier}` state badge on the card).

### Alternatives or additional context

- Today: linking an existing Linear issue is only possible at workspace-creation time; creating a new Linear issue is only possible on the Tasks page (and doesn't create/attach a workspace). GitHub issue/PR, by contrast, can be added/edited anytime from the workspace details dialog.
- The underlying pieces already exist (workspaces carry Linear linkage fields; a Linear create/update client is already in use for board-status→Linear sync and composer linking) — this is primarily about exposing link/create in the workspace update surface.
- Related: #3835 (sync worktree board status to linked Linear issue state).