feat(ide): upgrade codewhale-vscode from scaffold to a full Runtime API agent client (IDE stage 2)

[Hmbown]

Part of the ratified IDE program (founder 2026-09-02; see codewhale-ops PRODUCT_PRD §6 "IDE direction" and TRUTH_LEDGER 2026-09-02). Stage 2 of attach-then-fork: make Codewhale a first-class agent option inside existing editors.

Current state

extensions/vscode is a scaffold: openTerminal, startRuntime, checkRuntime commands, agent/snapshot webviews, Runtime API port/token settings (src/runtime.ts, default port 7878), and deliberately owns no background process.

Deliver

• Thread list + streaming chat panel over the Runtime API (POST /v1/threads, POST /v1/threads/{id}/turns, SSE GET /v1/threads/{id}/events).
• Approval prompts wired to POST /v1/approvals/{approval_id} and POST /v1/user-input/{thread_id}/{input_id}.
• Diff review of agent edits using the native VS Code diff editor.
• Provider/route/cost surface from /v1/providers and /v1/runtime/info (the apps control plane mirrors a Continue-shaped /v1/ide/status; keep fields compatible).
• Attach-or-spawn contract mirroring the desktop shell's spawn spec (codewhale app-server --http --host 127.0.0.1 --port <ephemeral> + runtime token, dedicated process group — see codewhale-apps clients/desktop-shell/src-tauri/src/local_runtime.rs RuntimeLaunchSpec).
• Distribute on Open VSX only (Microsoft Marketplace ToS excludes non-Visual Studio products; never publish there).

Acceptance

• Listed on Open VSX; install + connect to a local runtime without manual config beyond port/token.
• One end-to-end task: prompt → streaming turns → tool/approval flow → diff review, all inside VS Code.
• Tests cover the Runtime API client paths used.