fix: keep state mutations on one file

[danyel117]

Summary

• capture OPENCODE_GOAL_STATE_PATH once per queued mutation so reads and writes cannot target different files
• dispose every V1 and V2 plugin instance from test teardown, including after assertion failures
• add a deterministic regression that changes the state path between read and write
• restore Bun's native sequential runner while retaining centralized normal/coverage scripts

Root cause

An in-flight continuation from a prior test could begin a state mutation against fixture A. readStateEffect read A, but writeStateEffect resolved the environment variable again and wrote the old state into fixture B after the next test changed OPENCODE_GOAL_STATE_PATH. Captured failures showed objectives from earlier tests inside the current test's newly-created path.

The regression was observed RED before the fix (original objective remained in A and updated state moved to B) and GREEN after capturing one path for the mutation.

Validation

• fixed full suite: 0/50 loops
• after V2 teardown hardening: 0/20 additional loops
• bun run lint
• bun run typecheck
• bun run test (213 pass)
• bun run test:coverage (213 pass)
• bun run build
• bun run pack:dry-run
• Claude Opus review: no merge blocker