refactor(components): extract useDraftPersistence hook

[saagpatel]

What

Pulls the three draft-persistence callbacks out of `DraftTab.tsx` into a dedicated `useDraftPersistence` hook:

• `handleSaveDraft` — the big one. Builds the payload, dispatches to `saveDraft` (new) or `updateDraft` (existing), handles the runbook-scope rekey from `workspace:*` to `draft:`, logs the quality snapshot, and surfaces the success/warning toast.
• `handleConfirmOpenDraft` — save-and-open vs replace flow for pending draft dialogs.
• `handleConfirmOpenSimilarCase` — save-and-open vs replace vs compare flow for pending similar-case dialogs.

Shell reduction: 1893 → 1738 LOC (-155 on this PR).

Why

`handleSaveDraft` was the last remaining ~100-LOC callback in the shell. It was flagged as the biggest high-risk extraction because it reads 20+ pieces of shell state and writes 5 setters, but the two confirm-open callbacks that depend on it sit right next to it in the same concern — bundling all three keeps them co-located in one testable unit.

How

1. Create `useDraftPersistence.ts` that takes the saveable state, the `useWorkspaceDraftState`-derived appliers (`applyLoadedDraft`, `loadSimilarCaseIntoWorkspace`), the runbook-scope/saved-draft-id setters, and the toast callbacks as a single options object.
2. Move the three callbacks into the hook, preserving the exact payload shape, logging, and runbook-scope migration rules.
3. Update `DraftTab.tsx` to destructure the three handlers from the hook call.
4. Drop the `shouldMigrateVisibleRunbookSession` + `shouldProceedAfterSaveAttempt` imports from the shell — they now live inside the hook.
5. Add `useDraftPersistence.test.ts` with six `renderHook` tests covering the empty-workspace guard, new-draft path, existing-draft path, draft-open confirm flow (no pending and replace), and similar-case compare flow.

Testing

• `pnpm tsc --noEmit` — clean.
• `pnpm test` — 220 pass (was 214; +6 new hook tests).
• `pnpm ui:gate:static` — exit 0.

Risk / Notes

• `DraftTabHandle` unchanged (`saveDraft` still points at `handleSaveDraft`, now from the hook).
• All payload fields preserved: input/response/ticket/sources/case_intake/handoff_summary/model/autosave flag.
• Runbook-scope rekey + migration path preserved exactly, including the `runbookScopeLinked` fall-through that degrades the success toast to an advisory error when the migration fails.
• `saveCaseOutcome`-linked ordering is unchanged: `setAutosaveDraftId(null)` fires before `setSavedDraftId` so autosave state is cleared before the new id lands.

Wave 5 status after this PR

• DraftTab: 2573 → 1738 LOC (-835, ~32% total reduction)
• Tests: 184 → 220 (+36 direct Draft concern tests)
• 9 hooks + 1 panel extracted + 1 handle-shape test file
• Still above the 1000-LOC success target; see the four-PR roadmap for the remaining path.