Tasks board: first-class GitHub Issues provider (gh-auth, per-repo)

[Legacynnn]

Summary

Makes GitHub issues a first-class, natively-editable provider on the Tasks board (previously Linear-only), scoped per-repository and authed through the bundled gh CLI. This is the board-first half of the design (spec Phases 1–3); the context-sidebar work (Linear-in-sidebar + GitHub-issue inbox rebuild) is a planned Phase 4 follow-up.

What changed

Backend — multi-provider integrations layer

• Refactored the Linear-only integrations backend behind a TaskProvider trait + resolve_provider dispatch; Linear is now one impl, GitHub the other. Every IPC command routes through the trait (no behavior change for Linear).
• New dedicated integrations/github/ client over the existing forge::github gh-auth + GraphQL plumbing (one canonical GitHub auth path): repos-as-teams, issue listing, labels/assignees, create/update — split into client/map/ops/queries/responses modules.
• GitHub status model: Open / Done / Not planned mapped onto the normalized lifecycle, with open/close/reopen (+ close reason) write-back. Labels editable (absolute set), single assignee, title/body. Priority/project are N/A and ignored.
• Correctness: all GraphQL Value reads index through the {data} envelope; post-mutation reloads use an uncached path (avoids a stale-read-after-edit bug); repo labels/assignees are fully paginated; create_issue honors the target column.
• gh login resolution avoids a per-account profile network fan-out and is deterministic.
• Registered list_task_labels/list_task_assignees in the companion dispatch (pre-existing gap surfaced by tests).

Frontend — GitHub on the board

• IntegrationProvider = "linear" | "github"; provider tab enabled; board driven by persisted provider state.
• Provider-aware gating: GitHub drops priority/project facets + detail/create editors; provider-aware empty-state onboarding (gh-auth, no API key).
• Settings → Integrations gets a GitHub card (connect via existing gh sign-in, repository selector, refresh/disconnect/last-synced).

Test plan

• Rust: 15 suites pass; new unit tests cover GitHub status/issue mapping. cargo clippy --all-targets -D warnings clean.
• Sidecar: 429 pass.
• Frontend: tasks + settings suites green; new tests for facet gating + detail-view GitHub gating. (7 failures in editor/terminal are pre-existing on the base branch, unrelated.)
• biome clean.
• Manual smoke (needs a gh-authed env): connect GitHub → pick repo → Sync → verify columns, drag open↔closed, inline edits, create, agent-review, start-workspace; confirm Linear tab unchanged.

🤖 Generated with Claude Code

[Legacynnn]

Closing as superseded — already merged into main.

Every commit in this PR was relanded on main (matching headlines: baseline Linear tasks, TaskProvider trait + LinearProvider, GraphQL client wrapper, GitHub integration card, companion list_task_labels/assignees registration, etc.), and main has since advanced with the Phase 4 follow-up (Linear-in-sidebar inbox, GitHub issue-inbox search, integrations/github/{auth,inbox,map,ops,responses}.rs, extracted github-integration-card.tsx).

The merge conflicts were all add/add against these already-present files, and the net diff origin/main..HEAD is +205 / −20,038 — i.e. merging this branch would revert ~20k lines of newer work. Nothing to salvage; the feature shipped and evolved past this branch.