Issues panel + coordinator extension: host shared quadraui Board (Phase 0, read-only)

[JDonaghy]

Summary

Add an Issues activity-bar panel + coordinator extension to vimcode that hosts the
shared quadraui::Board component, sources its data from coord board --json, and routes
board actions to coord subcommands. This is Phase 0 (read-only board) of the
coordinator integration: the goal is that the full pipeline can eventually be driven from
vimcode at parity with coord-tui.

Design doc: docs/COORDINATOR_INTEGRATION.md (§3, §6, §7, §11).

Why

vimcode and coord-tui are both quadraui apps. We want users who live in either one to do
everything from it. coordinator owns the issue lifecycle (the verb); vimcode owns the code
(the noun). Hosting the board in vimcode is the first step toward the real differentiator —
reviewing the agent's work as real code, in a real editor (Phase 2+, follow-on issues).

Blocks / depends on

• quadraui — reusable Board component (render + input; BoardModel in / BoardAction
  out). JDonaghy/quadraui (filed alongside).
• claude-coordinator — coord board --json projection. JDonaghy/claude-coordinator
  (filed alongside).

Scope (Phase 0 — read-only)

Follow the platform-neutrality rule: shared logic in render.rs/engine, 1–3 lines of
wiring per backend, no bespoke GTK/TUI board code.

• src/render.rs — BoardData view model built from the coord board --json payload,
  handed to quadraui::Board; new ScreenLayout.board slot (model on ext_sidebar).
• src/core/ — engine fields for the panel (selection, focus, last fetched model, poll
  receiver). Pure; no Python/GTK. New src/core/coord_client.rs isolates the coord …
  subprocess + JSON parse so core stays testable (mock the client in tests).
• src/gtk/ + src/tui_main/ — register the Issues entry in the activity bar (the SC
  and Extensions panels are the template — TuiPanel, ext_sidebar, PanelRegistration)
  and draw quadraui::Board. Key/click → BoardAction (no actions wired yet in Phase 0
  beyond selection/open).
• Poll loop — refresh coord board --json on a timer using the existing
  poll_ext_registry / poll_sc_diff pattern (try_recv on a background receiver).
• Lua extension bundle — packaging/glue only: registers the Issues activity entry and
  :Coord* commands. Does not render the board and holds no pipeline logic.

Out of scope (later phases, separate issues)

• Actions (dispatch / test / review / merge) wired to coord subprocess — Phase 0 is
  read-only.
• Issue authoring as markdown buffers (:CoordRefine) — Phase 1.
• In-editor diff review tab + coord report-result round-trip — Phase 2 (the differentiator).
• Running coord notify from vimcode — opt-in, later.

Acceptance

• Selecting Issues in the activity bar renders the coordinator board (shared quadraui
  component) in both the TUI and GTK backends.
• The board reflects coord board --json and refreshes on the poll timer.
• No board-specific drawing code in src/gtk/ or src/tui_main/ beyond activity-bar
  registration + a quadraui::Board draw call.
• cargo build, cargo test --no-default-features, cargo clippy -- -D warnings,
  cargo fmt all pass.

Related

• Design doc: docs/COORDINATOR_INTEGRATION.md
• quadraui Board component, coordinator coord board --json (filed alongside).

[JDonaghy]

Part of the coordinator↔vimcode integration (design: vimcode/docs/COORDINATOR_INTEGRATION.md). Sibling pre-reqs: quadraui Board component JDonaghy/quadraui#362 · coord JSON projection JDonaghy/code-coordinator#550 · vimcode Issues panel #521.