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.test.ts

@@ -65,11 +65,31 @@ describe('proposalsFor', () => {
     const props = proposalsFor({ issue: 2, estimate: 'est/5d', priority: 'p/1' }, ['est/2d', 'p/3'], 'ChangeSource')
     expect(props).toHaveLength(2)
     expect(props[0].change).toEqual({ kind: 'reestimate', issue: 2, estimate: 'est/5d' })
-    expect(describeChange(props[0].plan)).toBe('est/2d → est/5d')
+    expect(props[0].summary).toBe('est/2d → est/5d')
     expect(props[1].change).toEqual({ kind: 'reprioritize', issue: 2, priority: 'p/1' })
     expect(props[0].issueTitle).toBe('ChangeSource')
   })
 
+  it('proposes an assignment only when it differs from the current assignee', () => {
+    const change = proposalsFor({ issue: 2, assignee: 'christian' }, [], undefined, { currentAssignee: null })
+    expect(change).toHaveLength(1)
+    expect(change[0].change).toEqual({ kind: 'assign', issue: 2, assignee: 'christian' })
+    expect(change[0].summary).toBe('assign → @christian')
+    expect(change[0].plan).toBeUndefined()
+    // already assigned → no proposal
+    expect(proposalsFor({ issue: 2, assignee: 'christian' }, [], undefined, { currentAssignee: 'christian' })).toEqual([])
+  })
+
+  it('proposes a milestone move, labeled from the milestones list, skipping a no-op', () => {
+    const ctx = { currentMilestoneId: 1, milestones: [{ id: 3, title: 'P5 — Dogfood' }] }
+    const props = proposalsFor({ issue: 2, milestone: 3 }, [], undefined, ctx)
+    expect(props).toHaveLength(1)
+    expect(props[0].change).toEqual({ kind: 'remilestone', issue: 2, milestone: 3, milestoneTitle: 'P5 — Dogfood' })
+    expect(props[0].summary).toBe('milestone → P5 — Dogfood')
+    // same milestone → no proposal
+    expect(proposalsFor({ issue: 2, milestone: 1 }, [], undefined, ctx)).toEqual([])
+  })
+
   it('drops a noop axis (already at the requested value)', () => {
     const props = proposalsFor({ issue: 2, estimate: 'est/2d', priority: 'p/1' }, ['est/2d', 'p/3'])
     expect(props.map((p) => p.change.kind)).toEqual(['reprioritize']) // estimate unchanged