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/gitea/client.ts

@@ -101,6 +101,8 @@ export interface GiteaClient {
   getIssueTimeline(index: number): Promise<LifecycleEvent[]>
   /** Every label defined on the repo (id + name), for name→id resolution. */
   listLabels(): Promise<GiteaLabel[]>
+  /** Repo collaborators (login + display name) — the assignable people. */
+  listCollaborators(): Promise<{ login: string; name: string }[]>
   /** Replace an issue's entire label set with the given label ids. Write. */
   setIssueLabels(index: number, labelIds: number[]): Promise<void>
   /** Replace an issue's assignees (empty array unassigns); returns the updated issue. Write. */
@@ -233,6 +235,19 @@ export function createGiteaClient(config: GiteaConfig, fetchImpl: FetchLike): Gi
       await request(`/issues/${index}/labels`, { method: 'PUT', body: { labels: labelIds } })
     },
 
+    async listCollaborators() {
+      const raw = await requestAll<{ login: string; full_name?: string }>(
+        (page) => `/collaborators?page=${page}&limit=${PAGE_LIMIT}`,
+      )
+      const collabs = raw.map((u) => ({ login: u.login, name: u.full_name || u.login }))
+      // The owner is assignable but is not returned by /collaborators — prepend them
+      // so a solo-owner repo still has an assignable person.
+      if (!collabs.some((c) => c.login === config.owner)) {
+        collabs.unshift({ login: config.owner, name: config.owner })
+      }
+      return collabs
+    },
+
     async setIssueAssignees(index, logins) {
       const raw = (await request(`/issues/${index}`, { method: 'PATCH', body: { assignees: logins } })) as RawIssue
       return normalizeIssue(raw)