Fix legacy Pinet assignment progress visibility

[tmustier]

Summary

• treat legacy unknown task assignments as implementation-like for progress reporting instead of suppressing them as read-only work
• resolve repo-less legacy issue rows against the current cwd repo so closed stale issues can be hidden
• keep closed/unmerged PR warnings visible while the implementation issue remains open

Follow-up to #751 / fixes remaining review concerns for #747.

Tests

• pnpm exec vitest run slack-bridge/task-assignments.test.ts slack-bridge/pinet-mesh-ops.test.ts slack-bridge/broker/helpers.test.ts
• pnpm lint
• pnpm typecheck
• pnpm test