feat(acp): client docs, agent-loop smoke scorecard, remove BackgroundAgentPool

[Patel230]

Summary

Three backlog items from the control-plane roadmap, stacked on #176:

1. Remove BackgroundAgentPool (deprecation → removal). The type,
   constructors, FormatResults, and reexport shims are gone. Zero production
   callers existed; Session.SpawnController() + taskruntime.Registry is the
   path. Deletes internal/engine/agent/background_agent.go (+ its test).
2. ACP client docs — docs/acp/client.md: wire protocol, worked lifecycle
   example (session/new → setMode → setIsolation → status → prompt),
   and a stdlib-only reference client. Matches the real JSON shapes (structured
   prompt array, hawk snapshot).
3. Benchmark scorecard — hawk eval smoke: headless agent-loop smoke
   benchmark. Drives the real Session.Stream loop with a stub provider (no
   API key, CI-safe) and scores steps / tool calls / token usage per task.
   Fixtures: smoke-read-file (≥1 Read tool call), smoke-no-tools
   (clean termination). internal/feature/eval/{smoke,headless,smoke_test}.go.

Test plan

• go test ./internal/feature/eval/ ./internal/engine/ ./internal/engine/agent/ ./internal/acp/
• go build ./... && go vet ./internal/feature/eval/ ./cmd/
• go run ./cmd/hawk eval smoke → 2/2 pass
• Pre-commit hooks (fmt/lint/imports) green