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.ts

@@ -17,6 +17,15 @@ import {
 export type IssueChange =
   | { kind: 'reestimate'; issue: number; estimate: EstimateLabel | null }
   | { kind: 'reprioritize'; issue: number; priority: PriorityLabel | null }
+  | { kind: 'assign'; issue: number; assignee: string | null }
+  | { kind: 'remilestone'; issue: number; milestone: number | null; milestoneTitle?: string | null }
+
+/** The label axes are the only kinds planned as a label swap; the rest are field writes. */
+export function isLabelChange(
+  change: IssueChange,
+): change is Extract<IssueChange, { kind: 'reestimate' | 'reprioritize' }> {
+  return change.kind === 'reestimate' || change.kind === 'reprioritize'
+}
 
 export interface LabelPlan {
   /** The full resulting label-name set (order: kept labels, then the new axis label). */
@@ -29,7 +38,10 @@ export interface LabelPlan {
   noop: boolean
 }
 
-function axisFor(change: IssueChange): { labels: readonly string[]; target: string | null } {
+/** A change that resolves to a label swap (the only kind `planIssueChange` accepts). */
+export type LabelChange = Extract<IssueChange, { kind: 'reestimate' | 'reprioritize' }>
+
+function axisFor(change: LabelChange): { labels: readonly string[]; target: string | null } {
   return change.kind === 'reestimate'
     ? { labels: ESTIMATE_LABELS, target: change.estimate }
     : { labels: PRIORITY_LABELS, target: change.priority }
@@ -40,7 +52,7 @@ function axisFor(change: IssueChange): { labels: readonly string[]; target: stri
  * change's axis except the target, adds the target if absent. Setting the axis
  * to null clears it. Cleans up a duplicated axis (two `est/*`) as a side effect.
  */
-export function planIssueChange(current: string[], change: IssueChange): LabelPlan {
+export function planIssueChange(current: string[], change: LabelChange): LabelPlan {
   const { labels: axis, target } = axisFor(change)
   const onAxis = current.filter((l) => axis.includes(l))
   const removed = onAxis.filter((l) => l !== target)
@@ -58,6 +70,26 @@ export function describeChange(plan: LabelPlan): string {
   return `${from} → ${to}`
 }
 
+/**
+ * One human-readable confirm line for ANY change kind — the unified describe for
+ * the propose-approve UI. Label kinds delegate to {@link describeChange} (so they
+ * need the issue's current labels); assign/milestone describe the field write
+ * directly. Pure; no network.
+ */
+export function summarizeChange(change: IssueChange, currentLabels: string[] = []): string {
+  switch (change.kind) {
+    case 'reestimate':
+    case 'reprioritize':
+      return describeChange(planIssueChange(currentLabels, change))
+    case 'assign':
+      return change.assignee ? `assign → @${change.assignee}` : 'unassign'
+    case 'remilestone':
+      return change.milestone == null
+        ? 'remove from milestone'
+        : `milestone → ${change.milestoneTitle ?? `#${change.milestone}`}`
+  }
+}
+
 /** A change the agent proposes: the concrete op + its diff, ready for approve-then-apply. */
 export interface ChangeProposal {
   change: IssueChange