apply_changes UI: assign + milestone in the dialog and Reginald's tool (#24)

Surfaces the assign/milestone mutations end-to-end so they're usable in-app and
by the agent — the rest of #24.

Agent path:
- propose_change tool + system prompt now accept `assignee` (login/null) and
  `milestone` (id/null). ProposeChangeArgs + proposalsFor extended: a new
  ProposalContext (current assignee/milestone + milestones list) lets a proposal
  skip no-ops and label the milestone. ChangeProposal gains an always-present
  `summary` (plan is now label-only) — chat-panel, use-chat, and the model
  executor render `summary`, so non-label proposals display correctly.

Dialog path:
- Client `listCollaborators()` (prepends the repo owner — /collaborators omits
  them, so a solo-owner repo still has an assignable person). New
  `gitea:collaborators` bridge. The Adjust dialog gains Assignee + Milestone
  pickers (current values from the reconciled backlog); pending assign/remilestone
  changes flow through the existing apply path.

Tests: +4 core (assign/milestone proposals with no-op skip; collaborators
owner-prepend + no-double-add). 138 core green; core + desktop typecheck clean;
14 fixture e2e green; live-backlog now drives the pickers on real data.
Fixed stale P2 refs in live-backlog (P2 is shipped → correctly off the runway).

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

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

@@ -90,18 +90,32 @@ export function summarizeChange(change: IssueChange, currentLabels: string[] = [
   }
 }
 
-/** A change the agent proposes: the concrete op + its diff, ready for approve-then-apply. */
+/** A change the agent proposes: the concrete op + its human diff, ready for approve-then-apply. */
 export interface ChangeProposal {
   change: IssueChange
-  plan: LabelPlan
+  /** One-line diff for the approve UI — always present ({@link summarizeChange}). */
+  summary: string
+  /** The label plan, for label kinds only (absent for assign/milestone). */
+  plan?: LabelPlan
   issueTitle?: string
 }
 
-/** What the `propose_change` tool accepts — a target issue and the axes to set. */
+/** What the `propose_change` tool accepts — a target issue and the fields to set. */
 export interface ProposeChangeArgs {
   issue: number
   estimate?: EstimateLabel
   priority?: PriorityLabel
+  /** Login to assign, or null to unassign. Omit to leave assignee untouched. */
+  assignee?: string | null
+  /** Milestone id to set, or null to clear. Omit to leave the milestone untouched. */
+  milestone?: number | null
+}
+
+/** Current state + lookups a proposal needs to skip noops and label the milestone. */
+export interface ProposalContext {
+  currentAssignee?: string | null
+  currentMilestoneId?: number | null
+  milestones?: { id: number; title: string }[]
 }
 
 function isEstimate(v: unknown): v is EstimateLabel {
@@ -112,25 +126,33 @@ function isPriority(v: unknown): v is PriorityLabel {
 }
 
 /**
- * Build the concrete, non-noop proposals for a `propose_change` request against
- * an issue's current labels. Invalid or unchanged axes are dropped — the agent
- * proposes only real changes, and never a label outside the est/* · p/* axes.
+ * Build the concrete, non-noop proposals for a `propose_change` request. Label
+ * axes (est/*, p/*) are validated and never invented; assign/milestone are
+ * emitted only when they differ from the issue's current value (via `ctx`). Each
+ * proposal carries a human `summary`; label kinds also carry the label plan.
  */
 export function proposalsFor(
   args: ProposeChangeArgs,
   currentLabels: string[],
   issueTitle?: string,
+  ctx: ProposalContext = {},
 ): ChangeProposal[] {
   const out: ChangeProposal[] = []
-  if (isEstimate(args.estimate)) {
-    const change: IssueChange = { kind: 'reestimate', issue: args.issue, estimate: args.estimate }
+  const pushLabel = (change: LabelChange) => {
     const plan = planIssueChange(currentLabels, change)
-    if (!plan.noop) out.push({ change, plan, issueTitle })
+    if (!plan.noop) out.push({ change, plan, summary: describeChange(plan), issueTitle })
   }
-  if (isPriority(args.priority)) {
-    const change: IssueChange = { kind: 'reprioritize', issue: args.issue, priority: args.priority }
-    const plan = planIssueChange(currentLabels, change)
-    if (!plan.noop) out.push({ change, plan, issueTitle })
+  if (isEstimate(args.estimate)) pushLabel({ kind: 'reestimate', issue: args.issue, estimate: args.estimate })
+  if (isPriority(args.priority)) pushLabel({ kind: 'reprioritize', issue: args.issue, priority: args.priority })
+
+  if (args.assignee !== undefined && args.assignee !== (ctx.currentAssignee ?? null)) {
+    const change: IssueChange = { kind: 'assign', issue: args.issue, assignee: args.assignee }
+    out.push({ change, summary: summarizeChange(change), issueTitle })
+  }
+  if (args.milestone !== undefined && args.milestone !== (ctx.currentMilestoneId ?? null)) {
+    const milestoneTitle = ctx.milestones?.find((m) => m.id === args.milestone)?.title ?? null
+    const change: IssueChange = { kind: 'remilestone', issue: args.issue, milestone: args.milestone, milestoneTitle }
+    out.push({ change, summary: summarizeChange(change), issueTitle })
   }
   return out
 }