fix(desktop): recover failed beta servers and clean unused CLI stages

[kitlangton]

Why

Follow-up to #57309 based on review findings:

1. With version: undefined on beta, decide() in @opencode/client/service never replaced a failed (500) or timed-out (503) server from an older beta build.
2. Skipping cleanStages when adopting an existing server left older ~200 MB staged CLI directories accumulating under userData/cli/<version>-<arch>.

What Changes

• Retry Service.ensure once with version: cli.version when an unpinned beta ensure fails with "Background service failed to start" or "Timed out waiting for the background service to start", while keeping 404 protocol incompatibility as a startup error.
• Always run cleanStages on initial launch when the running server's version is known from file ?? client.fallback(), passing keep: keptStages(cli.version, runningVersion) so both the bundled stage and the running server's stage stay on disk while older stages are removed. If reading the registry fails (runningVersion undefined), skip stage cleanup so an adopted server's stage is never deleted.
• Keep reconnect using serviceVersion(...) (undefined) so auto-reconnects (for example after sleep/wake or SSE drop) never replace a healthy adopted server. Follow-up: add an explicit in-app restart button and an API revision handshake on /api/info to detect protocol skew directly.
• Simplify sidecar-probe.ts to const version = CHANNEL === "beta" ? undefined : bundledVersion(); if (version === "") return. Note for rebasing with feat(desktop): preserve Drive-managed development state #57112: background-service.ts should use drive ? undefined : serviceVersion(...) when those lines meet.
• Trim cli-version.test.ts to 33 lines (serviceVersion, keptStages, and shouldRetryEnsure tables).

Decision Table

Channel	Registered service state (/api/info)	Version vs bundled CLI	Outcome
beta	Healthy (200, { version, pid })	Older, matching, or newer	Adopt (reuse), keep bundled + running stages, clean older stages
beta	Starting (503, { version, pid })	Older, matching, or newer	Wait (wait), then adopt; if startup times out, retry with cli.version (replace)
beta	Failed (500, { version, pid })	Older or newer	Initial fail retries once with cli.version → replace (clear)
beta	Missing, unreachable, wrong PID, or 3x timeout	Any	Start bundled CLI (replace / spawn) and clean unused stages
beta	Incompatible protocol (404 on /api/info)	Any	Preserve running server and show in-app startup error (localServerStartup)
prod (stable)	Healthy (200, { version, pid })	Matching	Adopt (reuse)
prod (stable)	Healthy, starting, failed, or 404	Older or newer	Replace with bundled CLI (replace)

Scope

• packages/client/src/promise/service.ts
• packages/desktop/src/main/service/cli-version.ts
• packages/desktop/src/main/service/cli-version.test.ts
• packages/desktop/src/main/service/cli-stages.ts
• packages/desktop/src/main/service/cli-stages.test.ts
• packages/desktop/src/main/service/desktop-cli.ts
• packages/desktop/src/main/service/sidecar-probe.ts
• packages/desktop/src/main/service/background-service.ts

Verification

• bun test src/main/service/cli-version.test.ts src/main/service/cli-stages.test.ts src/main/service/background-service-state.test.ts in packages/desktop
• bun run typecheck in packages/desktop and packages/client
• bun run lint:changed origin/beta