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>

apps/desktop/src/main/gitea.ts

@@ -19,6 +19,7 @@ import {
   type GiteaConfig,
   type GiteaLabel,
   type IssueChange,
+  isLabelChange,
   type LifecycleEvent,
   makeDirectiveEntry,
   parseCapacityConfig,
@@ -263,19 +264,32 @@ export function registerGiteaIpc(): void {
     return client.getIssue(index)
   })
 
-  // The write path (apply_changes). Additive label swaps, applied only after the
-  // renderer's propose-approve. Returns the plan + the freshly-read issue.
+  // The write path (apply_changes), applied only after the renderer's propose-
+  // approve. Label swaps (est/p) return the plan; field writes (assign, milestone)
+  // return the freshly-read issue directly. Either way the snapshot is invalidated
+  // so the board + forecast reflect the change.
   ipcMain.handle('gitea:applyChange', async (_event, change: IssueChange) => {
     const client = getGiteaClient()
     if (!client) return { ok: false as const, reason: 'unconfigured' as const }
-    const current = await client.getIssue(change.issue)
-    const plan = planIssueChange(current.labels, change)
-    if (plan.noop) return { ok: true as const, plan, issue: current }
-    const ids = await resolveLabelIds(client, plan.labels)
-    await client.setIssueLabels(change.issue, ids)
-    const issue = await client.getIssue(change.issue)
-    invalidateSnapshot() // the board + forecast must reflect the label change
-    return { ok: true as const, plan, issue }
+
+    if (isLabelChange(change)) {
+      const current = await client.getIssue(change.issue)
+      const plan = planIssueChange(current.labels, change)
+      if (plan.noop) return { ok: true as const, plan, issue: current }
+      const ids = await resolveLabelIds(client, plan.labels)
+      await client.setIssueLabels(change.issue, ids)
+      const issue = await client.getIssue(change.issue)
+      invalidateSnapshot()
+      return { ok: true as const, plan, issue }
+    }
+
+    // Field writes — the client returns the updated issue.
+    const issue =
+      change.kind === 'assign'
+        ? await client.setIssueAssignees(change.issue, change.assignee ? [change.assignee] : [])
+        : await client.setIssueMilestone(change.issue, change.milestone)
+    invalidateSnapshot()
+    return { ok: true as const, issue }
   })
 
   // capture_work filing: open each approved issue with its est/* + p/* labels.