feat(doc-mode): + New contract sidebar header + + New ephemeral task per folder (#121)

The unified surface didn't expose any way to create a new contract or a
new ephemeral task — the only paths were `/directives/<id>?document=...`
or right-click context menus that no longer existed in the documents-per-
directive sidebar restructure. Two new affordances:

* **Sidebar header gains a "+ New" button** that opens NewContractModal
  (title + goal + optional repository_url). On submit calls
  `useDirectives.create` and navigates the user into the new directive.
  Header label updated from "Documents" to "Contracts" so the button's
  intent reads naturally.

* **Each open directive folder gets a "+ New ephemeral task" button**
  alongside the existing "+ New document" affordance. Opens
  NewEphemeralTaskModal (name + plan), calls the existing
  `createDirectiveTask` API, navigates the user into the new task's
  transcript at `?task=<id>`.

NewContractModal and NewEphemeralTaskModal are local to the page —
mirror the styling of the older NewTaskModal that lived here pre-
restructure. ⌘/Ctrl+Enter submits.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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