Speed up targeted opencode tests

[kitlangton]

Summary

• Add lightweight benchmark/profile scripts for packages/opencode test-suite research.
• Reduce avoidable test setup in slow plugin, provider, workspace, SDK, config, and HTTP listener tests.
• Record measured kept/discarded experiments in perf/test-suite.md.

Measurements

• Final single full-suite benchmark: 225.069s via bun run bench:test.
• Focused wins include httpapi-provider 7.905s -> 2.980s, TUI plugin lifecycle 7.330s -> 1.507s, SDK parity 8.011s -> 5.180s, workspace 12.949s -> 8.305s.

Verification

• bunx prettier --check ...
• file-scoped bunx oxlint ... (0 errors; existing warnings remain)
• bun typecheck from packages/opencode
• focused changed tests: bun run test -- test/plugin/install-concurrency.test.ts test/server/httpapi-listen.test.ts test/control-plane/workspace.test.ts test/config/config.test.ts test/server/httpapi-sdk.test.ts test/provider/provider.test.ts test/server/httpapi-provider.test.ts test/cli/tui/plugin-lifecycle.test.ts
• bun run bench:test
• pre-push bun turbo typecheck