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>
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