feat: the write path — apply estimate/priority changes to gitea (P4-3 apply_changes)

The first write path. Read, forecast, and calibration were all real; now you can
*manage* CommiTea from CommiTea. Estimates/priority are exclusive label axes, so
a change is a label swap — proposed, approved, then written. Nothing is assumed.

core (@commitea/core):
- planIssueChange(current, change): pure diff planner — swaps the est/*|p/* axis,
  clears on null, dedups a doubled axis; returns the resulting label set + a
  before/after diff + noop flag. describeChange() renders "est/2d → est/5d".
- request() seam extended for writes (method/body, JSON, 204). client gains
  listLabels() (name→id) and setIssueLabels() (PUT /issues/{n}/labels).

app:
- main bridge gitea:applyChange — resolves plan.labels → ids (cached, refetch on
  miss), PUTs, returns the plan + fresh issue. Token never leaves main.
- preload + global.d.ts expose applyChange; useBacklog returns a refetch so a
  write re-reconciles the board + forecast.
- Issue screen: an Adjust button (shown only when configured) opens a
  propose-approve Dialog — estimate/priority pickers, live "est/3d → est/8d"
  consequence, Apply/Cancel. AppShell wires it, reflects new labels on the open
  issue immediately, and refetches.

Verified: 83 core tests green (7 apply-changes + 2 client-write new), desktop
typecheck clean, 14 fixture e2e green. Live spec exercises propose + CANCEL (no
mutation); the real PUT was verified once manually (change #2 est/3d→est/8d→200,
reverted clean). Icon: pencil (no sliders-horizontal in the set).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/desktop/e2e/live-backlog.spec.ts

@@ -45,6 +45,21 @@ test.describe('live backlog', () => {
     await expect(win.getByText(/cold-start · \d+\/20|curve active · n ≥ 20/)).toBeVisible()
     await win.screenshot({ path: join(here, '.artifacts', 'screens', 'live-calibration.png'), fullPage: true, animations: 'disabled' })
 
+    // Write path (apply_changes): open the top scheduled issue, propose an
+    // estimate change, and confirm the propose-approve diff renders — then CANCEL
+    // so the live run never mutates the real repo (the PUT is unit-tested; a
+    // one-off change→revert verified it against gitea manually).
+    await rail.getByRole('button', { name: 'Morning service' }).click()
+    await win.getByRole('main').getByRole('link').first().click()
+    await expect(win.getByText(/· stephen\/commitea/)).toBeVisible()
+    await win.getByRole('button', { name: 'Adjust' }).click()
+    await expect(win.getByText('Adjust estimate & priority')).toBeVisible()
+    await win.getByRole('combobox').first().selectOption('est/8d')
+    await expect(win.getByText('Proposed label change')).toBeVisible()
+    await expect(win.getByText(/est\/8d/).last()).toBeVisible()
+    await win.screenshot({ path: join(here, '.artifacts', 'screens', 'live-apply-change.png'), fullPage: true, animations: 'disabled' })
+    await win.getByRole('button', { name: 'Cancel' }).click() // no mutation
+
     await app.close()
   })
 })