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>

apps/desktop/src/main/gitea.ts

@@ -333,6 +333,17 @@ export function registerGiteaIpc(): void {
     }
   })
 
+  // Assignable people (repo collaborators) for the Adjust dialog's assignee picker.
+  ipcMain.handle('gitea:collaborators', async () => {
+    const client = getGiteaClient()
+    if (!client) return []
+    try {
+      return await client.listCollaborators()
+    } catch {
+      return []
+    }
+  })
+
   // ---- config (team onboarding) ----
   ipcMain.handle('config:get', () => publicConfig())