refactor(instance): remove ambient instance context

[kitlangton]

Summary

• Remove the legacy LocalContext export from project/instance-context.ts now that instance context is carried via InstanceRef.
• Stop CLI bootstrap from installing unused instance ALS around callbacks.
• Convert the affected workspace tests from the old Promise-style withInstance harness to it.instance, so tests use the same Effect instance fixture instead of ambient context.

Verification

• bunx prettier --write packages/opencode/src/project/instance-context.ts packages/opencode/src/cli/bootstrap.ts packages/opencode/test/control-plane/workspace.test.ts
• bun typecheck from packages/opencode
• bun run test -- test/control-plane/workspace.test.ts from packages/opencode
• bunx oxlint packages/opencode/src/project/instance-context.ts packages/opencode/src/cli/bootstrap.ts packages/opencode/test/control-plane/workspace.test.ts (0 errors; existing warnings in workspace.test.ts)
• pre-push root bun turbo typecheck