fix(client): surface managed startup stderr - #41793

What

Surface actionable stderr when a managed background-service contender exits unsuccessfully. This lets opencode2 service start report the existing port-conflict guidance instead of reducing it to an exit code.

Closes #41696

Before / After

Before: serve --service emitted the actionable port-conflict diagnostic, but Service.ensure spawned it with stdio: "ignore". The managed client saw only Server process exited with code 1 and could continue spawning contenders without exposing the cause.

After: managed contenders continuously retain only the final 8 KiB of stderr. Once a contender closes with a nonzero exit or signal, its startup error includes that tail, so the configured-port remediation reaches the caller.

How

• packages/client/src/effect/service.ts captures a bounded stderr tail and waits for child close so output is drained before formatting failures.
• packages/client/src/promise/service.ts mirrors the same behavior without adding an Effect runtime dependency.
• The stderr pipe and child remain unrefed so a successful detached service does not keep the client alive.
• Client fixtures verify tail bounding for both APIs; the CLI regression exercises a real child process through the managed Service.ensure boundary using the same actionable conflict text.

Scope

• Does not change service election, contender limits, spawn cadence, or retry policy.
• Does not address other startup-failure presentation tracked by V2: critical startup failures should show actionable errors #35158.
• Does not change the server-side port-conflict diagnostic, which was already actionable.

Testing

• cd packages/client && bun run test test/service.test.ts test/promise-service.test.ts (21 passed)
• cd packages/cli && bun run test test/service.test.ts --test-name-pattern "managed service startup reports an actionable port conflict|unrelated managed port occupancy reports an actionable conflict" (2 passed)
• cd packages/client && bun typecheck
• cd packages/cli && bun typecheck
• bunx prettier --check packages/client/src/effect/service.ts packages/client/src/promise/service.ts packages/client/test/service.test.ts packages/client/test/promise-service.test.ts packages/client/test/fixture/service.ts packages/cli/test/service.test.ts
• Push hook: bun turbo typecheck --concurrency=3 (33 tasks passed)

Flow

sequenceDiagram
    participant Client as Service.ensure
    participant Child as serve --service contender
    Client->>Child: spawn detached with stderr pipe
    Child-->>Client: stderr chunks
    Client->>Client: retain final 8 KiB
    Child-->>Client: close with nonzero exit
    Client-->>Client: append stderr tail to startup error