Delegate existing goal milestones while keeping completion manual

[wangtaotaotao95]

Closes #72.

Goal milestones can now open the existing delegation sheet with their goal and milestone context. The task keeps a stable milestoneId, appears under that milestone with its real status, and opens the existing task/artifact detail. A successful task leaves the milestone unchecked for the user to complete explicitly; it does not append a duplicate completed milestone. Goal-only tasks retain their existing behavior.

Creation validates goal ownership and milestone membership. The sheet reuses a request ID after a failed submission, while a later explicit delegation gets a new task. Deleted milestones stay deleted and task detail explains the missing link. Checkbox updates now change only the selected milestone in one database statement, preserving current titles, ordering, other completion states and concurrent additions. Goal status updates likewise no longer write a stale full goal snapshot.

Validation on Node 24.21.0 / pnpm 11.19.0:

• All 281 tests pass, including ownership/membership rejection, concurrent request replay, separate later delegation, rename/reorder/deletion, concurrent manual progress, successful/failed/cancelled/review-blocked outcomes, legacy goal tasks, and persisted results after database restart.
• pnpm lint, pnpm typecheck, pnpm --dir apps/worker typecheck, pnpm build:server, pnpm build:web, pnpm build:ios, pnpm build:android pass.
• Chromium sample-workspace walkthrough: delegate an existing milestone to the finance workflow, open its successful result, verify it remains unchecked with no duplicate milestone, and manually complete only that milestone. Checked desktop and phone-sized Web layouts. Rich Threads provisioning was stubbed; finance execution and milestone APIs used the real local server/database.
• Native simulator/device runtime and live provider paths were not tested. iOS/Android checks above are bundle exports, not native runtime validation.

Screenshots (fictional sample data; review assets are kept outside the source branch):

AI assistance: implemented and verified with Codex.