apply_changes: unify the mutation tool with assign + milestone (#24)

The write path handled only the est/* and p/* label axes. Extend it to the two
mutations dogfooding proved were missing — assignee and milestone — behind one
unified change model:

- IssueChange gains `assign` and `remilestone` kinds. `planIssueChange` is now
  typed to LabelChange (the label kinds only); `isLabelChange` narrows, and a new
  pure `summarizeChange(change, currentLabels?)` gives one confirm line for ANY
  kind (label kinds delegate to describeChange; field writes describe directly).
- GiteaClient gains `setIssueAssignees` / `setIssueMilestone` (PATCH /issues/{n},
  returning the updated issue; milestone 0 clears).
- The main `gitea:applyChange` handler branches: label swaps resolve ids +
  setIssueLabels as before; field writes call the new client methods. Snapshot
  invalidated either way so board + forecast catch up. ApplyChangeResult.plan is
  now optional (absent for field writes).
- issue-screen uses summarizeChange for its diff preview.

Tests: +5 covering isLabelChange, and summarizeChange for assign/unassign,
milestone set-by-title / by-id / removal, and label delegation. Full core suite
132 green; core + desktop typecheck clean.

Follow-up (rest of #24): surface assignee/milestone pickers in the Adjust dialog
and extend the agent's propose_change tool so Reginald can propose them too.

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

packages/core/src/changes/apply-changes-v0.test.ts

@@ -1,6 +1,13 @@
 import { describe, expect, it } from 'vitest'
 
-import { describeChange, type IssueChange, planIssueChange, proposalsFor } from './apply-changes-v0.js'
+import {
+  describeChange,
+  isLabelChange,
+  type IssueChange,
+  planIssueChange,
+  proposalsFor,
+  summarizeChange,
+} from './apply-changes-v0.js'
 
 describe('planIssueChange', () => {
   it('swaps the estimate label, keeping non-axis labels', () => {
@@ -77,3 +84,31 @@ describe('proposalsFor', () => {
     expect(proposalsFor({ issue: 2 }, ['est/2d'])).toEqual([])
   })
 })
+
+describe('unified change model (assign + milestone)', () => {
+  it('isLabelChange narrows label kinds only', () => {
+    expect(isLabelChange({ kind: 'reestimate', issue: 1, estimate: 'est/2d' })).toBe(true)
+    expect(isLabelChange({ kind: 'reprioritize', issue: 1, priority: 'p/1' })).toBe(true)
+    expect(isLabelChange({ kind: 'assign', issue: 1, assignee: 'christian' })).toBe(false)
+    expect(isLabelChange({ kind: 'remilestone', issue: 1, milestone: 3 })).toBe(false)
+  })
+
+  it('summarizeChange describes an assignment and an unassignment', () => {
+    expect(summarizeChange({ kind: 'assign', issue: 1, assignee: 'christian' })).toBe('assign → @christian')
+    expect(summarizeChange({ kind: 'assign', issue: 1, assignee: null })).toBe('unassign')
+  })
+
+  it('summarizeChange describes a milestone set (by title) and removal', () => {
+    expect(summarizeChange({ kind: 'remilestone', issue: 1, milestone: 3, milestoneTitle: 'P5 — Dogfood' })).toBe(
+      'milestone → P5 — Dogfood',
+    )
+    expect(summarizeChange({ kind: 'remilestone', issue: 1, milestone: 3 })).toBe('milestone → #3')
+    expect(summarizeChange({ kind: 'remilestone', issue: 1, milestone: null })).toBe('remove from milestone')
+  })
+
+  it('summarizeChange delegates label kinds to describeChange (needs current labels)', () => {
+    const change: IssueChange = { kind: 'reestimate', issue: 1, estimate: 'est/5d' }
+    expect(summarizeChange(change, ['est/2d'])).toBe('est/2d → est/5d')
+    expect(summarizeChange(change, ['est/5d'])).toBe('no change')
+  })
+})