Dashboard backlog: show assignee alongside reporter (#700)

[waleedkadous]

Summary

Adds the issue assignee to dashboard backlog rows. Each row now shows both the reporter (r:) and assignee (a:) on the same line:

r: @waleedkadous  a: none
r: @waleedkadous  a: @amr
r: @waleedkadous  a: @amr, @bob

Closes #700.

Changes

Touched the four layers that needed plumbing for the new assignees field:

1. packages/codev/src/lib/forge-contracts.ts — added assignees?: Array<{ login: string }> to IssueListItem.
2. packages/codev/scripts/forge/github/issue-list.sh — added assignees to the gh issue list --json flag (gitlab/gitea presets unchanged — they already return all fields by default).
3. packages/codev/src/agent-farm/servers/overview.ts — added assignees?: string[] to BacklogItem and mapped issue.assignees?.map(a => a.login) in deriveBacklog. Empty/missing arrays become undefined.
4. packages/types/src/api.ts — mirrored the assignees?: string[] field on OverviewBacklogItem.
5. packages/dashboard/src/components/BacklogList.tsx — renders r: @<reporter> a: @<assignee> (or a: none when unassigned, comma-separated for multiple). Also added a CSS rule so the new span shares the muted style of the reporter span.

Out of scope

• No filtering or sorting by assignee (display only, per the issue).
• Team page and other views untouched.

Tests

Added explicit coverage for the three required cases (none / one / many) at both layers:

• Backend (packages/codev/src/agent-farm/__tests__/overview.test.ts): three new deriveBacklog cases — single assignee, multiple assignees, and missing/empty arrays staying undefined.
• UI (packages/dashboard/__tests__/BacklogList.test.tsx, new file): four cases — a: none, single @login, comma-separated multiple, and empty array treated as none.

All 133 backend overview tests and the 4 new BacklogList tests pass. Full codev build is green.

Review

Decisions worth flagging

• BacklogItem.assignees is left undefined when the array would be empty, mirroring the existing convention for author. Components see one shape (undefined or non-empty array), and the a: none rendering is decided at the view layer.
• r: is only rendered when an author exists (matching the prior behavior — the existing item.author && ... guard is preserved). a: always renders to keep the visual format unambiguous.
• The gh script change is GitHub-only — gitlab/gitea presets emit their CLIs' default JSON, which already includes assignees on the items they return. If those presets need normalization, that's a follow-up beyond this issue.

Test plan

• pnpm --filter @cluesmith/codev-types build
• pnpm --filter @cluesmith/codev-core build
• pnpm --filter @cluesmith/codev build (green, including dashboard build)
• Backend: npx vitest run src/agent-farm/__tests__/overview.test.ts (133/133)
• UI: npx vitest run BacklogList (4/4)
• Visual sanity check in running dashboard (architect to verify)

[waleedkadous]

Excellent work — the plumbing is clean across all four layers and the tests cover the three cases I asked for (none / one / many). I particularly like that you added assignees to the gh --json flag so the data is actually fetched, and that you treated empty-array and missing-array identically.

Approved to merge.