ci: serialize stateful tests

[danyel117]

Summary

• run the stateful Bun suite through a globally single-test scheduler
• centralize normal and coverage commands as bun run test and bun run test:coverage
• use those scripts in PR CI, publishing, and contributor documentation

Root cause

The tests isolate state with OPENCODE_GOAL_STATE_PATH, but that environment variable, fake system time, and the state mutation queue are process-global. During a failing plan-agent test, instrumentation captured another test's objective (native retry must win) in its state. This made timing assertions and goal lifecycle assertions nondeterministic.

--max-concurrency 1 alone did not stop cross-file interference on Bun 1.3.8. Routing all tests through the concurrent scheduler and limiting it to one did.

Validation

• plain runner reproduced failures in 2/12 full-suite loops
• V2-only runner reproduced failures in 2/50 loops
• fixed runner passed 20/20 full-suite loops
• bun run lint
• bun run typecheck
• bun run test (212 pass)
• bun run test:coverage (212 pass)
• bun run build
• bun run pack:dry-run
• Claude Opus review: no merge blocker