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/agent/agent-tools.ts

@@ -38,15 +38,19 @@ export const QUERY_PROJECT_TOOL: ToolDecl = {
 export const PROPOSE_CHANGE_TOOL: ToolDecl = {
   name: 'propose_change',
   description:
-    "Propose an estimate and/or priority change to an issue. This does NOT apply anything — it shows the " +
-    'human a diff to approve. Use it whenever the user asks to re-estimate or reprioritize. After calling it, ' +
-    "tell the user you've *proposed* the change for approval — never say it is done.",
+    'Propose a change to an issue — estimate, priority, assignee, and/or milestone. This does NOT apply ' +
+    'anything; it shows the human a diff to approve. Use it whenever the user asks to re-estimate, reprioritize, ' +
+    'assign someone (or unassign), or move an issue to a milestone. Pass a login for `assignee` (null to ' +
+    'unassign) and a milestone id for `milestone` (null to remove). After calling it, tell the user you have ' +
+    '*proposed* the change for approval — never say it is done.',
   parameters: {
     type: 'object',
     properties: {
       issue: { type: 'number', description: 'the issue number to change' },
       estimate: { type: 'string', enum: ['est/1d', 'est/2d', 'est/3d', 'est/5d', 'est/8d'] },
       priority: { type: 'string', enum: ['p/1', 'p/2', 'p/3', 'p/4'] },
+      assignee: { type: 'string', description: 'gitea login to assign, or null to unassign' },
+      milestone: { type: 'number', description: 'milestone id to set, or null to remove from its milestone' },
     },
     required: ['issue'],
   },
@@ -84,7 +88,7 @@ export const REGINALD_SYSTEM = [
   'You are Reginald, the calm, dry project manager inside CommiTea — a tool that runs projects on Gitea.',
   'Call query_project to ground every answer in the real project; never invent issues, numbers, or dates.',
   'The scheduler and forecasts are deterministic code — report their output, do not recompute it.',
-  'To change an estimate or priority, call propose_change — it shows the human a diff to approve.',
+  'To change an estimate, priority, assignee, or milestone, call propose_change — it shows the human a diff to approve.',
   'When the PM states standing intent ("pilots first", "freeze scope"), call record_directive to log it.',
   'Never claim a change is applied; you propose, the human approves. Forecasts are ranges, never single dates.',
   'Refer to issues as #<number>. Be brief and plain — a sentence or two. No preamble, no bullet dumps.',